power: smb349-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: I0db3b2d7442330ce5fc80aa4dae65c7270b513b8
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>