ARM: config: Switch PXA27x platforms to use PXA RTC driver

With the SA1100 and PXA RTC drivers be mutually exclusive and no
longer sharing hardware, PXA27x/PXA3xx platforms must use the PXA RTC
driver as the SA1100 platform device is no longer registered.

This change should be almost transparent to userspace. Former users of
pxa-rtc should be aware that 2 RTCs will be available on their kernels,
rtc0 being sa1100-rtc and rtc1 being pxa-rtc. Any userspace relying on
the fact that rtc0 was pxa-rtc should be fixed.

As a consequence:
 - the first reboot after the switch will have the wrong time,
 - on dual boot platform where the other OS programs some logic into the
   sa1100 rtc IP, a lack of fix in userspace, ie. a kernel changing
   sa1100-rtc thinking it is pxa-rtc could have dire consequence, such
   as wiping the other OS data partition.

(Thanks to Robert Jarmik for help on the above commit text.)

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Daniel Mack <daniel@zonque.org>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Sergey Lapin <slapin@ossfans.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Mike Rapoport <mike@compulab.co.il>
Cc: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>