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/malta64el_defconfig b/configs/malta64el_defconfig
index 7d11966..55f624b 100644
--- a/configs/malta64el_defconfig
+++ b/configs/malta64el_defconfig
@@ -11,6 +11,7 @@
 CONFIG_TARGET_MALTA=y
 CONFIG_BUILD_TARGET="u-boot-swap.bin"
 CONFIG_CPU_MIPS64_R2=y
+CONFIG_SYS_MIPS_TIMER_FREQ=250000000
 CONFIG_SYS_LITTLE_ENDIAN=y
 # CONFIG_AUTOBOOT is not set
 CONFIG_BOARD_EARLY_INIT_F=y