rtc: pl031: convert the driver to driver model

With this patch, PL031 driver is converted to driver-model-compliant
driver. In addition, CONFIG_SYS_RTC_PL031_BASE is no longer valid.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>