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/malta64_defconfig b/configs/malta64_defconfig
index f6bb2d0..054c5e6 100644
--- a/configs/malta64_defconfig
+++ b/configs/malta64_defconfig
@@ -10,6 +10,7 @@
 CONFIG_ENV_ADDR=0xFFFFFFFFBE3E0000
 CONFIG_TARGET_MALTA=y
 CONFIG_CPU_MIPS64_R2=y
+CONFIG_SYS_MIPS_TIMER_FREQ=250000000
 # CONFIG_AUTOBOOT is not set
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_MISC_INIT_R=y