arm: aarch64: add Xen virtual machine support

This commit adds minimal support for running zephyr as Xen guest. It
does not use xen PV console, which is somewhat hard to implement, as it
depends on xenbus infrastructure. Instead SBSA-compatible PL011 uart is
used.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>