power: qpnp-fg-gen3: make CHARGE_FULL property writable by user

CHARGE_FULL property indicates the learnt capacity of the battery
from the last capacity learning cycle. However, in some rare
cases, if the learnt capacity is having an unexpected value, it
requires a reset to a good value which is not possible for an
user who cannot reinsert a battery or do an explicit write to FG
SRAM. Allowing CHARGE_FULL to be writable helps this.

Change-Id: I05aa8392f103685f8fc1ba5a3780122150be0ee6
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>