hwmon: add max1111_read_channel() for use by sharpsl_pm

This is not generic, and is added here for backward compatibility.
It is made an individual commit here to make it easier for revert
once the sharpsl_pm gets generic enough.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>