rtc: stmp3xxx: Get rid of mach-specific accessors

Replace the accessors with standard readl/writel to remove their
platform-dependency. Also, drop __raw_(read|write)l-accessors while we
are here.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Tested-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>