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/ap121_defconfig b/configs/ap121_defconfig
index 1f4a793..77d2d1f 100644
--- a/configs/ap121_defconfig
+++ b/configs/ap121_defconfig
@@ -11,6 +11,7 @@
 CONFIG_DEBUG_UART_BOARD_INIT=y
 CONFIG_SYS_LOAD_ADDR=0x81000000
 CONFIG_ARCH_ATH79=y
+CONFIG_SYS_MIPS_TIMER_FREQ=200000000
 CONFIG_DEBUG_UART=y
 CONFIG_SYS_MEMTEST_START=0x80100000
 CONFIG_SYS_MEMTEST_END=0x83f00000