drm/xe: add 28-bit address support in struct xe_reg

Xe driver currently supports 22-bit addresses for MMIO access.
Future platforms will have additional MMIO extension with
larger address spaces, and to access them, the driver will
have to support wider address representation.
Please note that while the XE_REG macro is used for MMIO access,
XE_REG_EXT macro will be used for MMIO-extension access.

Signed-off-by: Koby Elbaz <kelbaz@habana.ai>
Reviewed-by: Ofir Bitton <obitton@habana.ai>
Reviewed-by: Moti Haimovski <mhaimovski@habana.ai>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>