ARM: Samsung: Make uart_save static in pm.c file

Fixes the following sparse warning:
arch/arm/plat-samsung/pm.c:77:21:
warning: symbol 'uart_save' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>