power: qpnp-charger: make the battery capacity writeable

There are some applications that may want to fake a battery state of
charge (SoC). To standarize this on different devices, make the battery
power supply capacity property writeable.

Change-Id: I75fd301febb994032de8b07c1d58bb38ffdc2c55
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>