iio: qcom-spmi-adc5: add SMB_TEMP channel for PMIC7

Add support to read SMB_TEMP channel on PM8350B that is output by
charge pump (smb139x). When this virtual channel is read for
one of SMB139x charger pumps, PBS does the configuration for that
SMB139x appropriately.

Change-Id: Ia9af1ef78571c56898057d567117fcd9b7d28172
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>