ARM: SAMSUNG: Save/restore only selected uart's registers

Basically this code gets executed only during debugging i.e when
DEBUG_LL & SAMSUNG_PM_DEBUG is on, so required only for UART used
for debugging. Since we are removing static iodesc entries for UARTs,
so now only the selected (CONFIG_DEBUG_S3C_UART) UART will be
ioremapped by the debug_ll_io_init() for DEBUG_LL, so save/restore
uart registers only for selected uart.

Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>