xen/riscv: implement virt_to_maddr()

xen/riscv: implement virt_to_maddr()

Implement the virt_to_maddr() function to convert virtual addresses
to machine addresses. The function includes checks for valid address
ranges, specifically the direct mapping region (DIRECTMAP_VIRT_START)
and the Xen's Linkage (XEN_VIRT_START) region. If the virtual address
falls outside of these regions, an assertion will trigger.
To implement this, the phys_offset variable is made accessible
outside of riscv/mm.c.

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>