New MRRM tables can have subtables that are larger than 255 bytes.
Add a new header typedef that uses u16 for Length. Could be
backported to acpi_aspt_header, struct acpi_dmar_header, struct acpi_nfit_header,
struct acpi_prmt_module_header, struct acpi_prmt_module_info. Will be used for
upcoming ERDT table.
MRRM table has a 26-byte reserved section in header. Add ACPI_DMT_BUF26
to describe this in struct acpi_dmtable_info.