MIPS: Fix typos in arch/mips/Kbuild.platforms

Commit 26bff9eb ("MIPS: Only include the platform file needed")
misspelled "txx9" to "tx99", so fix it.

Fixes: 26bff9eb ("MIPS: Only include the platform file needed")
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>