soc: arm64: xenvm: move Kconfig option to soc

The XEN_INITIAL_DOMAIN option was defined at board level, however, some
drivers such as the XEN serial driver has dependencies on
XEN_INITIAL_DOMAIN, meaning any new board has to define its own copy of
XEN_INITIAL_DOMAIN. Move the option to SoC Kconfig.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>