msm: 8974: add gpiomux configuration for the external 9x25 modem

For fusion3.5, the external mdm9x25 modem is powered up and
monitored by gpios. The platform is identified by the subtype
value set in the CDT.

Change-Id: I401f39041dfcb2187c726eb13bc4ea932e651e88
Signed-off-by: Joel King <joelking@codeaurora.org>