Move zynqmp_ipi.h to platform specific include directory.
Rename it to plat_ipi.h instead of platform name. So, it can
be used to common source files which needs platform specific
data.

Signed-off-by: Tejas Patel <tejas.patel@xilinx.com>
Reviewed-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Jolly Shah <jollys@xilinx.com>