drivers: soc: xilinx: fix compile warning by defining payload array length to 32

In zynqmp-ipi-mailbox.c and zynqmp_r5_remoteproc.c, the variables of
zynqmp_ipi_message structures are all using 32 as the maximum size of
data length. So move IPI_BUF_LEN_MAX to zynqmp-ipi-message.h to share with
zynqmp_power.c.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>