ARM: dts: AM33xx: Add PRU mboxes to mailbox node

The mailbox DT node data is updated with additional mbox data to
communicate with the PRU0 (User Id 1) and PRU1 (User Id 2) cores
within the PRU-ICSS subsystem. The following are the assignments
of mailbox fifos between MPU and the remote PRU cores:

		MPU Tx/PRU Rx	MPU Rx/PRU Tx
	PRU0	2		3
	PRU1	4		5

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