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>

arch/mips/Kbuild.platforms

@@ -36,8 +36,8 @@ platform-$(CONFIG_SIBYTE_SB1250)	+= sibyte/
 platform-$(CONFIG_SIBYTE_BCM1x55)	+= sibyte/
 platform-$(CONFIG_SIBYTE_BCM1x80)	+= sibyte/
 platform-$(CONFIG_SNI_RM)		+= sni/
-platform-$(CONFIG_MACH_TX39XX)		+= tx99/
-platform-$(CONFIG_MACH_TX49XX)		+= tx99/
+platform-$(CONFIG_MACH_TX39XX)		+= txx9/
+platform-$(CONFIG_MACH_TX49XX)		+= txx9/
 platform-$(CONFIG_MACH_VR41XX)		+= vr41xx/
 
 # include the platform specific files