drivers/rtc/rtc-pl031.c: fix the missing operation on enable

drivers/rtc/rtc-pl031.c: fix the missing operation on enable

The RTC control register should be enabled in the process of
initializing.

Without this patch, I failed to enable RTC in Hisilicon Hi3620 SoC.  The
register mapping section in RTC is always read as zero.  So I doubt that
ST guys may already enable this register in bootloader.  So they won't
meet this issue.

Signed-off-by: Haojian Zhuang <[email protected]>
Cc: Srinidhi Kasagar <[email protected]>
Cc: Linus Walleij <[email protected]>
Cc: Alessandro Zummo <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>