soc: ti: pruss: Fix system suspend/MStandby config issues

soc: ti: pruss: Fix system suspend/MStandby config issues

The PRU-ICSS subsystem has a separate PRUSS_CFG module that contains
various configuration registers. This includes a control bit STANDBY_INIT
in 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 system
suspend/resume functionality on AM33xx/AM437x/AM57xx SoCs requires
all initiators to assert their MStandby signal properly inorder to
successfully enter suspend, and resume on a wakeup event.

Certain firmwares can enable the OCP master ports through the
STANDBY_INIT programming on the firmware side in order to access
peripherals or memories external to the PRUSS. This causes a hang
in the resume sequence on AM33xx/AM437x boards and requires a
board reset to come out of the hang.

This patch adds the preliminary System PM callbacks in the PRUSS SoC
bus driver, and fixes this system resume hang by setting the STANDBY_INIT
in the PM system suspend callback and resetting it back in the PM system
resume callback, if so configured. The clearing of the STANDBY_INIT
during resume requires an acknowledgment from PRCM and is done through
the monitoring of the PRUSS_SYSCFG.SUB_MWAIT bit.

NOTE:
1. This patch only adds the PM callbacks with code to fix the System
   Suspend/Resume hang issue on AM33xx/AM437x SoCs, but does not
   implement the full context save and restore required for the PRUSS
   drivers to work across system suspend/resume when the power domain
   is switched off (L4PER domain is switched OFF on AM335x/AM437x
   during system suspend/resume, so PRUSS modules do lose context).
2. The PRUSS driver functionality on AM57xx SoCs is not affected that
   much because the PER power domain to which the PRUSS IPs belong is
   not switched OFF during suspend/resume.

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