arm64: versal: RO multi_boot register in non JTAG bootmode

arm64: versal: RO multi_boot register in non JTAG bootmode

The main reason for this change is that upstream QEMU has no multiboot
register implemented that's why access to it fails which ends up in CI
failure for our target.
That's why in JTAG bootmode returns 0 which is correct behaviour because
multiboot register is not used in this mode and value should be ignored and
as a side effect it is also fixing CI/Qemu issue.

Also move versal_get_bootmode() to avoid function declaration.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/484b9cafc45e72308a1a29a3ab772020f96784cc.1736155238.git.michal.simek@amd.com