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/imgtec_xilfpga_defconfig b/configs/imgtec_xilfpga_defconfig
index c275990..a0632bc 100644
--- a/configs/imgtec_xilfpga_defconfig
+++ b/configs/imgtec_xilfpga_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SYS_PROMPT="MIPSfpga # "
 CONFIG_SYS_LOAD_ADDR=0x80500000
 CONFIG_TARGET_XILFPGA=y
+CONFIG_SYS_MIPS_TIMER_FREQ=50000000
 CONFIG_MIPS_CACHE_SETUP=y
 CONFIG_MIPS_CACHE_DISABLE=y
 # CONFIG_MIPS_BOOT_ENV_LEGACY is not set