SPEAr: explicitly select clk src for UART

UART in u-boot intends to run on 48MHz clock supplied by USB PLL.
Explicitly select the intended clock source.

Signed-off-by: Shiraz Hashim <shiraz.hashim@st.com>
Signed-off-by: Amit Virdi <amit.virdi@st.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>