drivers/rtc/rtc-palmas.c: support for backup battery charging

Palmas series device like TPS65913, TPS80036 supports the backup battery
for powering the RTC when no other energy source is available.

The backup battery is optional, connected to the VBACKUP pin, and can be
nonrechargeable or rechargeable.  The rechargeable battery can be charged
from the system supply using the backup battery charger.

Add support for enabling charging of this backup battery.  Also add the DT
binding document and the new properties to have this support.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Acked-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>