soc: ti: pruss: add helper function to enable OCP master ports

The PRU-ICSS subsystem on OMAP-architecture based SoCS (AM33xx, AM437x
and AM57xx SoCs) has a control bit STANDBY_INIT in the PRUSS_CFG register
to initiate a Standby sequence (when set) and trigger a MStandby request
to the SoC's PRCM module. This same bit is also used to enable the OCP
master ports (when cleared). The clearing of the STANDBY_INIT bit requires
an acknowledgment from PRCM and is done through the monitoring of the
PRUSS_SYSCFG.SUB_MWAIT bit.

Add a helper function pruss_cfg_ocp_master_ports() to allow the PRU
client drivers to control this bit and enable or disable the firmware
running on PRU cores access to any peripherals or memory to achieve
desired functionality. The access is disabled by default on power-up
and on any suspend (context is not maintained).

Signed-off-by: Suman Anna <s-anna@ti.com>