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_serval_defconfig b/configs/mscc_serval_defconfig
index b5d0190..055ed68 100644
--- a/configs/mscc_serval_defconfig
+++ b/configs/mscc_serval_defconfig
@@ -12,6 +12,7 @@
 CONFIG_ARCH_MSCC=y
 CONFIG_SOC_SERVAL=y
 CONFIG_DDRTYPE_H5TQ1G63BFA=y
+CONFIG_SYS_MIPS_TIMER_FREQ=208333333
 CONFIG_SYS_MEMTEST_START=0x80000000
 CONFIG_SYS_MEMTEST_END=0x87c00000
 CONFIG_SYS_LITTLE_ENDIAN=y