rtc/m41t62: Add support for M41T82 with HT (Halt Update)

Add support for the M41T82 RTC to the m41t62 driver. The only
difference that needs to be handled by this driver, is to
clear the HT (Halt Update) bit upon reset. This bit is not
used on the M41T62, so its save to clear this bit always.

The M41T82 support will be used by the X600 (SPEAr600)
board support.

Signed-off-by: Stefan Roese <sr@denx.de>