Driver for Zipit Z2 battery chip

This patch adds driver for Zipit Z2 battery chip called AER915. No
details are known about the chip. The chip is available through I2C bus
at address 0x55 and it's register 0x02 contains battery voltage.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>