power: smb5: Add support to read USBIN_I, USBIN_V, and charger temperature

Add driver support for reading USB input voltage and current, and
charger temperature for PM855B, which are obtained from VADC channels.
Also update the documentation to correct an error, add descriptions
for io-channels properties, and update the example device tree with
required properties.

CRs-Fixed: 2228622
Change-Id: Iaa230050b2c8c02a5dfbe0ed1de49c6b374074ed
Signed-off-by: Harry Yang <harryy@codeaurora.org>
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>