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/boston32r6el_defconfig b/configs/boston32r6el_defconfig
index 50032dd..67abbb9 100644
--- a/configs/boston32r6el_defconfig
+++ b/configs/boston32r6el_defconfig
@@ -10,6 +10,7 @@
 CONFIG_ENV_ADDR=0xBFFE0000
 CONFIG_TARGET_BOSTON=y
 CONFIG_CPU_MIPS32_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