uboot: zynqmp: allow to use custom psu_init files

uboot: zynqmp: allow to use custom psu_init files

U-Boot SPL configures pinmuxes, clocks and other low-level devices. On
the Xilinx ZynqMP SoCs the code to do this resides in a file called
psu_init_gpl.c which is initially generated by the Xilinx development
tools. Add an option to pass these files from the outside (e.g. in the
board files).

For this to work properly, a patch to U-Boot is needed. However this
patch must be applied by each defconfig using
BR2_TARGET_UBOOT_ZYNQMP_PSU_INIT_DIR. If it were in boot/uboot/ to be
applied unconditionally, it would break the build for configs using a
U-Boot version 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: add comment about U-Boot version requirements to use this
option.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>