ARM: mediatek: clean up mach-mediatek/Makefile

Kbuild descends into arch/arm/mach-mediatek/Makefile only when
CONFIG_ARCH_MEDIATEK is enabled.  So, obj-$(CONFIG_ARCH_MEDIATEK)
is always equivalent to obj-y in this Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>