ARM: S5PV310: Add serial port support

This patch adds UART serial port support for S5PV310.
In the case of that serial device has just one clock source, driver can not
control clock source. So add check function in get_clksrc and set_clksrc.

Signed-off-by: Jongpill Lee <boyko.lee@samsung.com>
Acked-by: Changhwan Youn <chaos.youn@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>