power: smb135x-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: Icb50b2aefe98bc2aa75d927624a13adf28b118c6
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>