MIPS: convert CONFIG_SYS_MIPS_TIMER_FREQ to Kconfig

This converts the following to Kconfig:
    CONFIG_SYS_MIPS_TIMER_REQ

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/configs/mscc_servalt_defconfig b/configs/mscc_servalt_defconfig
index d471654..6e225a6 100644
--- a/configs/mscc_servalt_defconfig
+++ b/configs/mscc_servalt_defconfig
@@ -11,6 +11,7 @@
 CONFIG_SYS_LOAD_ADDR=0x100000
 CONFIG_ARCH_MSCC=y
 CONFIG_SOC_SERVALT=y
+CONFIG_SYS_MIPS_TIMER_FREQ=250000000
 CONFIG_SYS_MEMTEST_START=0x80000000
 CONFIG_SYS_MEMTEST_END=0x9fc00000
 CONFIG_SYS_LITTLE_ENDIAN=y