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/linkit-smart-7688_defconfig b/configs/linkit-smart-7688_defconfig
index a129679..0bdb4e6 100644
--- a/configs/linkit-smart-7688_defconfig
+++ b/configs/linkit-smart-7688_defconfig
@@ -15,6 +15,7 @@
 CONFIG_ARCH_MTMIPS=y
 CONFIG_SOC_MT7628=y
 CONFIG_BOARD_LINKIT_SMART_7688=y
+CONFIG_SYS_MIPS_TIMER_FREQ=290000000
 CONFIG_MIPS_CACHE_SETUP=y
 CONFIG_MIPS_CACHE_DISABLE=y
 CONFIG_RESTORE_EXCEPTION_VECTOR_BASE=y