refactor(xilinx): rename platform function to generic name

Refactor two platform specific functions, plat_versal_get_mmap() and
plat_versal_net_get_mmap(), to use a more generic function name
plat_get_mmap(). The function can be used in the common code to
obtain  memory region.

Change-Id: I3eeb24aff217eef30af60a7742cbebe9d3b2edce
Signed-off-by: Prasad Kummari <prasad.kummari@amd.com>
10 files changed