MIPS: Malta: Register UP SMP ops if all else fails

If we fail to register any real SMP implementations, fall back to
registering the dummy UP implementation. Otherwise when we build an SMP
kernel & run it on a system where the SMP implementations fail to probe
(eg. QEMU) the kernel will perform a NULL dereference attempting to call
mp_ops->smp_setup() from plat_smp_setup().

Notably this fixes booting kernels with CPS SMP enabled on QEMU, which
doesn't currently implement the CM, CPC or GIC.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Rob Herring <robh@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: Markos Chandras <markos.chandras@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/11223/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -425,6 +425,7 @@ config MIPS_MALTA
 	select MIPS_L1_CACHE_SHIFT_6
 	select PCI_GT64XXX_PCI0
 	select MIPS_MSC
+	select SMP_UP if SMP
 	select SWAP_IO_SPACE
 	select SYS_HAS_CPU_MIPS32_R1
 	select SYS_HAS_CPU_MIPS32_R2

arch/mips/mti-malta/malta-init.c

@@ -302,6 +302,7 @@ void __init prom_init(void)
 		return;
 	if (!register_vsmp_smp_ops())
 		return;
+	register_up_smp_ops();
 }
 
 void platform_early_l2_init(void)