hwmon: add max1111 Low-power Multichannel Serial 8-bit ADCs

Driver based on corgi_ssp.c and sharpsl_pm.c, previously done by Richard
Purdie and many others.

Now changed to generic HWMON device and expose all the ADC input value
through sysfs.

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