MIPS: Fix undefined reference to physical_memsize

Since commit d41e6858 ("MIPS: Kconfig: Set default MIPS system type
as generic") switched the default platform to the "generic" platform,
allmodconfig has been failing with the following linker error (among
other errors):

arch/mips/kernel/vpe-mt.o In function `vpe_run':
(.text+0x59c): undefined reference to `physical_memsize'

The Lantiq platform already worked around the same issue in commit
9050d50e ("MIPS: lantiq: Set physical_memsize") by declaring
physical_memsize with the initial value of 0 (on the assumption that the
actual memory size will be hard-coded in the loaded VPE firmware), and
the Malta platform already provided physical_memsize.

Since all other platforms will fail to link with the VPE loader enabled,
only allow Lantiq and Malta platforms to enable it, by way of a
SYS_SUPPORTS_VPE_LOADER which is selected by those two platforms and
which MIPS_VPE_LOADER depends on. SYS_SUPPORTS_MULTITHREADING is now a
dependency of SYS_SUPPORTS_VPE_LOADER so that Kconfig emits a warning if
SYS_SUPPORTS_VPE_LOADER is selected without SYS_SUPPORTS_MULTITHREADING.

Fixes: d41e6858 ("MIPS: Kconfig: Set default MIPS system type as generic")
Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: John Crispin <john@phrozen.org>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: linux-mips@linux-mips.org
Tested-by: Guenter Roeck <linux@roeck-us.net>
Patchwork: https://patchwork.linux-mips.org/patch/18453/

arch/mips/Kconfig

@@ -390,6 +390,7 @@ config LANTIQ
 	select SYS_SUPPORTS_32BIT_KERNEL
 	select SYS_SUPPORTS_MIPS16
 	select SYS_SUPPORTS_MULTITHREADING
+	select SYS_SUPPORTS_VPE_LOADER
 	select SYS_HAS_EARLY_PRINTK
 	select GPIOLIB
 	select SWAP_IO_SPACE
@@ -517,6 +518,7 @@ config MIPS_MALTA
 	select SYS_SUPPORTS_MIPS16
 	select SYS_SUPPORTS_MULTITHREADING
 	select SYS_SUPPORTS_SMARTMIPS
+	select SYS_SUPPORTS_VPE_LOADER
 	select SYS_SUPPORTS_ZBOOT
 	select SYS_SUPPORTS_RELOCATABLE
 	select USE_OF
@@ -2282,9 +2284,16 @@ config MIPSR2_TO_R6_EMULATOR
 	  The only reason this is a build-time option is to save ~14K from the
 	  final kernel image.
 
+config SYS_SUPPORTS_VPE_LOADER
+	bool
+	depends on SYS_SUPPORTS_MULTITHREADING
+	help
+	  Indicates that the platform supports the VPE loader, and provides
+	  physical_memsize.
+
 config MIPS_VPE_LOADER
 	bool "VPE loader support."
-	depends on SYS_SUPPORTS_MULTITHREADING && MODULES
+	depends on SYS_SUPPORTS_VPE_LOADER && MODULES
 	select CPU_MIPSR2_IRQ_VI
 	select CPU_MIPSR2_IRQ_EI
 	select MIPS_MT