MIPS: jz4740: Fix Kbuild Platform file.

The platform specific files should be included via the platform-y
variable.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Patchwork: https://patchwork.linux-mips.org/patch/1719/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/jz4740/Platform

-core-$(CONFIG_MACH_JZ4740)	+= arch/mips/jz4740/
+platform-$(CONFIG_MACH_JZ4740)	+= jz4740/
 cflags-$(CONFIG_MACH_JZ4740)	+= -I$(srctree)/arch/mips/include/asm/mach-jz4740
 load-$(CONFIG_MACH_JZ4740)	+= 0xffffffff80010000