boot/uboot/uboot.mk: add pmufw.elf support

This patch adds support for zynqmp pmufw.elf files.
It will allow buildroot to use pmufw.elf binaries directly
from the Xilinx git repository built by petalinux in
addition to still supporting pmufw.bin binaries built
by the zynqmp-pmufw-builder.

https://github.com/Xilinx/ubuntu-firmware/tree/v2022.1_22.04_1/xlnx-firmware



Signed-off-by: Neal Frager <neal.frager@amd.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>