zynqmp: pm: Remove unused NODE_AFI, add NODE_EXTERN
zynqmp: pm: Remove unused NODE_AFI, add NODE_EXTERN

NODE_EXTERN is the slave node which represents an external wake
source.

Signed-off-by: Mirela Simonovic <789a56bf0be082732c408d9b08112674350ff427@aggios.com>
Acked-by: Will Wong <f41c7c74d20640e662309249fad8c7f26d607868@xilinx.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware