s5j/serial: retrieve UCLK freq from clock driver
s5j/serial: retrieve UCLK freq from clock driver

In S5J, source clock of UART can be one of following: OSCCLK and
WPLL_DIV12. Now that the clock driver is available, we may retrieve
clock frequency of the source clock, instead of using hard-coded clock
value.

Change-Id: Idc372a8c30cd214da545d40baf1eed9f723102d2
Signed-off-by: Heesub Shin <45471ea77bd08f3d26635f3f5adb40ba41250a2b@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT