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/maltael_defconfig b/configs/maltael_defconfig
index c1829ac..8b86d74 100644
--- a/configs/maltael_defconfig
+++ b/configs/maltael_defconfig
@@ -10,6 +10,7 @@
 CONFIG_ENV_ADDR=0xBE3E0000
 CONFIG_TARGET_MALTA=y
 CONFIG_BUILD_TARGET="u-boot-swap.bin"
+CONFIG_SYS_MIPS_TIMER_FREQ=250000000
 CONFIG_SYS_LITTLE_ENDIAN=y
 # CONFIG_AUTOBOOT is not set
 CONFIG_BOARD_EARLY_INIT_F=y