arm64: zynqmp: Add third backup bootmode

arm64: zynqmp: Add third backup bootmode

I found this issue when was running py/test.py on zcu102 which is for me by
default setup to SD boot mode without any way to change boot mode.
Alternative software bootmode selection to JTAG is not working because JTAG
mode is 0 which also reset value for it. That's why saying SPL to take
u-boot.itb from RAM instead of SD in SD boot mode is not possible via
alternative bootmode selection.
That's why setup third boot mode to JTAG(BOOT_DEVICE_RAM) as final
fallback.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>