core/driver: Add support for mapping the debug UART from the driver

core/driver: Add support for mapping the debug UART from the driver

If the debug UART is memory-mapped, we can only access prior to
switching to hypervisor mappings if the driver supports us in this. By
adding a debug_uart memory region to the system configuration, we tell
the driver about the mapping need. In turn, the driver reports the
virtual address via an additional header field. The mapping can be
released on Linux side right after enabling the hypervisor

Provided the virtual address of the UART mapping as chosen by Linux does
not conflict with our remapping region, this mapping can safely be
replicated into the hypervisor address space so that we don't need to
adjust the UART access after enabling our own mapping.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>