arm64: zynqmp: Support semihosting boot method

arm64: zynqmp: Support semihosting boot method

Currently, when we boot from JTAG we try to boot U-Boot from RAM.
However, this is a bit tricky to time, since the debugger has to wait
for SPL to initialize RAM before it can load U-Boot. This can result in
long waits, since occasionally initializing RAM (and other things in
psu_init) takes a long time to complete and the debugger must wait for
this worst case.

Support semihosting if it is enabled, as it lets U-Boot tell the
debugger when we are ready for the image. This means we don't have to
wait any more than necessary. We don't change the default config to
ensure we don't break compatibility with existing debuggers that don't
expect us to hit semihosting breakpoints.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Link: https://lore.kernel.org/r/20240222200512.2679830-1-sean.anderson@linux.dev
Signed-off-by: Michal Simek <michal.simek@amd.com>