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

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

commit e7e034e18a0ab6bafb2425c3242cac311164f4d6 upstream.

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 <haojian.zhuang@linaro.org>
Cc: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>