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/boston64r6_defconfig b/configs/boston64r6_defconfig
index 7f6c158..f6e246e 100644
--- a/configs/boston64r6_defconfig
+++ b/configs/boston64r6_defconfig
@@ -10,6 +10,7 @@
 CONFIG_ENV_ADDR=0xFFFFFFFFBFFE0000
 CONFIG_TARGET_BOSTON=y
 CONFIG_CPU_MIPS64_R6=y
+CONFIG_SYS_MIPS_TIMER_FREQ=30000000
 # CONFIG_MIPS_BOOT_CMDLINE_LEGACY is not set
 # CONFIG_MIPS_BOOT_ENV_LEGACY is not set
 CONFIG_MIPS_BOOT_FDT=y