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>