[acpi] Add acpi_ioremap() to map an ACPI-described address

[acpi] Add acpi_ioremap() to map an ACPI-described address

An ACPI Generic Address Structure (GAS) may be used to describe the
location of a peripheral such as an early boot console.  Add the
relevant definitions and provide acpi_ioremap() as a helper function
to map a region described using this structure.

Signed-off-by: Michael Brown <mcb30@ipxe.org>