extcon: rt8973a: Add Richtek RT8973A extcon driver

This patch add support for Richtek RT8973A which is Micro USB Switch OVP
and i2c interface. The RT8973A is a USB port accessory detector and switch
that is optimized to protect low voltage system from abnormal high input
voltage (up to 28V) and supports high speed USB operation. Also, RT8973A
support 'auto-configuration' mode. If auto-configuration mode is enabled,
RT8973A would control internal h/w patch for USB D-/D+ switching.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>