mx51evk: Fix 2 hours reset issue

The mx51evk u-boot has an issue that system will get reset
every 2 hours.

MC13892 has an inside charge timer which expires in 120 minutes.
If ICHRG and CHGAUTOB are not set properly, this timer expiration
will get system power recycled.

Since mx51evk has no Li-Ion battery on board, the patch sets
ICHRG in externally powered mode and sets CHGAUTOB bit to avoid
automatic charging, so that system will not get reset by this
timer expiration.

The patch also corrects the bit field definition of register 48
(Charger 0) per latest MC13892 Reference Manual.

Signed-off-by: Shawn Guo <shawn.gsc@gmail.com>