power: smb135x-charger: improve handling USB insertion/removal

Currently, we handle the USB insertion through source
detect interrupt and USB removal through under voltage interrupt.

In certain cases where the VBUS drops less than 5V (say 4.4 V)
and VBAT is at 4.3 V, PHY disconnects. It will never get
reasserted since the VBUS doesn't drop below 1V and goes
back to 5V.

To handle such corner cases, handle the USB removal for DCP
on the falling edge of UV interrupt and on the falling edge of
SRC_DET interrupt for CDP/SDP.

Change-Id: I061f781a1ecf9af79c014c4dd4993dd1c34470bf
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>