MIPS: fix duplicated slashes for Platform file path

platform-y accumulates platform names with a slash appended.
The current $(patsubst ...) ends up with doubling slashes.

GNU Make still include Platform files, but in case of an error,
a clumsy file path is displayed:

  arch/mips/loongson2ef//Platform:36: *** only binutils >= 2.20.2 have needed option -mfix-loongson2f-nop.  Stop.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>