boot/uboot: document that the zynqmp pmufw can be in ELF format

This feature was added in commit d07e6b7071cf ("boot/uboot/uboot.mk: add
pmufw.elf support"), document it in kconfig.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

boot/uboot/Config.in

@@ -492,6 +492,8 @@ config BR2_TARGET_UBOOT_ZYNQMP_PMUFW
 	  (e.g. http://...), and it will be downloaded and used from
 	  the download directory.
 
+	  The PMU firmware binary can be either in ELF or BIN format.
+
 	  If empty, the generated boot.bin will not contain a PMU
 	  firmware.