am57xx: Set tps659038 PMIC GPIO7 pad mux value to POWERHOLD

The GPIO7 pad mux should be programmed to POWERHOLD value
as per board design. In cases where the PMIC is shut off the
mux is set to GPIO7 mode. So during initialization to be on the
safer side set the mode to POWERHOLD.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>