xilinx: zynqmp: fix incorrect map not align with IPI HW

Current IPI module register description is not align with IPI HW. The
registers with the wrong offset are not used so it does not cause real
issues. This patch aligns the register description.

Additionally comments added to explain why recv function does not check
any flag prior copying rx data.

Fixes: 660b0c77 ("mailbox: zynqmp: ipi mailbox driver")
Signed-off-by: Ibai Erkiaga <ibai.erkiaga-elorza@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>