power: act8846_pmic: fix number of registers

The highest register on ACT8846 is 0xf5, so set the number of registers
to 0xf6, ensuring that the pmic read/write commands are able to access
all of the supported registers (and many that are not valid, since the
register space is quite sparse).

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>