Fix the following sparse warning:

arch/arm/mach-omap2/pm33xx-core.c:270:14: warning: symbol 'am43xx_get_rtc_base_addr' was not declared.

The am43xx_get_rtc_base_addr has only call site within pm33xx-core.c
It should be static

Fixes: 8c5a916f ("ARM: OMAP2+: sleep33/43xx: Add RTC-Mode support")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Samuel Zou <zou_wei@huawei.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>