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/comtrend_wap5813n_ram_defconfig b/configs/comtrend_wap5813n_ram_defconfig
index 0663418..79568cf 100644
--- a/configs/comtrend_wap5813n_ram_defconfig
+++ b/configs/comtrend_wap5813n_ram_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SYS_LOAD_ADDR=0x80100000
 CONFIG_ARCH_BMIPS=y
 CONFIG_SOC_BMIPS_BCM6368=y
+CONFIG_SYS_MIPS_TIMER_FREQ=200000000
 CONFIG_MIPS_CACHE_SETUP=y
 CONFIG_MIPS_CACHE_DISABLE=y
 # CONFIG_MIPS_BOOT_CMDLINE_LEGACY is not set