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/tplink_wdr4300_defconfig b/configs/tplink_wdr4300_defconfig
index b60f634..8728906 100644
--- a/configs/tplink_wdr4300_defconfig
+++ b/configs/tplink_wdr4300_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SYS_LOAD_ADDR=0xa1000000
 CONFIG_ARCH_ATH79=y
 CONFIG_BOARD_TPLINK_WDR4300=y
+CONFIG_SYS_MIPS_TIMER_FREQ=280000000
 CONFIG_SYS_MEMTEST_START=0x80100000
 CONFIG_SYS_MEMTEST_END=0x83f00000
 CONFIG_HAS_CUSTOM_SYS_INIT_SP_ADDR=y