platform/rdn2: set number of MHU channels for RD-N2-Cfg2

Set the number of MHU channels for SCP to SCP communication required
to pass information about PCIe topology discovery.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Change-Id: I12a2e51d45ca61a7fa434f5c6b1eaa17a6392c6b