For some SoCs, the system clock frequency may not equal to the
ARCH Timer's frequency.

This patch uses the CONFIG_TIMER_CLK_FREQ instead of
CONFIG_SYS_CLK_FREQ, then the system clock macro and arch timer
macor could be set separately and without interfering each other.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>