soc: ti: pruss: Add support for PRU-ICSS subsystems on AM57xx SoCs

The AM57xx family of SoCs supports two PRU-ICSS instances, each of
which has two PRU processor cores. The two PRU-ICSS instances are
identical to each other, and are very similar to the PRU-ICSS1 of
AM33xx/AM43xx except for a few minor differences like the RAM sizes
and the number of interrupts coming into the MPU INTC. They do
not have a programmable module reset line unlike those present on
AM33xx/AM43xx SoCs. The modules are reset just like any other IP
with the SoC's global cold/warm resets. Each PRU-ICSS's INTC is also
preceded by a Crossbar that enables multiple external events to be
routed to a specific number of input interrupt events. Any interrupt
event directed towards PRUSS needs this crossbar to be setup properly
on the firmware side.

The existing PRUSS platform driver has been enhanced to support
these AM57xx PRU-ICSS instances through new AM57xx specific
compatible for properly probing and booting all the different PRU
cores in each PRU-ICSS processor subsystem. A build dependency with
SOC_DRA7XX is also added to enable the driver to be built in
AM57xx-only configuration (there is no separate Kconfig option
for AM57xx vs DRA7xx).

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Grzegorz Jaszczyk <grzegorz.jaszczyk@linaro.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>