uboot: zynqmp: generate SPL image with PMUFW binary

In order to boot on the Xilinx ZynqMP SoCs, U-Boot SPL requires a
recent PMU firmware loaded. Instruct U-Boot to add pmufw.bin to the
boot.bin file together with U-Boot SPL, and the boot ROM will load
both.

For this to work properly, a patch to U-Boot is needed. However this
patch must be applied by each defconfig that wishes to use
BR2_TARGET_UBOOT_ZYNQMP_PMUFW. If it were in boot/uboot/ to be applied
unconditionally, it would break the build for configs using a U-Boot
version higher than 2018.7-rc1 where the patch is already applied.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Joel Carlson <joelsoncarl@gmail.com>
Reviewed-by: Joel Carlson <JoelsonCarl@gmail.com>
Tested-by: Joel Carlson <JoelsonCarl@gmail.com>
[Thomas:
 - indicate that this feature requires U-Boot 2018.07, since commit
   c7df098a71e05dc81cee818747759e8060b59626 is needed.
 - define UBOOT_ZYNQMP_KCONFIG_PMUFW only once, and instead use a
   variable named UBOOT_ZYNQMP_PMUFW_PATH to store the path to the PMU
   firmware]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>