gpio: qpnp-pin: Fix documentation for src-select binding

The documentation incorrectly specifies 'select' instead of
'src-select'. Update this to be consistent with the driver.

Change-Id: I90bffc26affdefa42192c7f10d4af80ad0340f7f
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>