[ioapi] Provide combined MMIO and port I/O accessors

[ioapi] Provide combined MMIO and port I/O accessors

Some devices (such as a 16550 UART) may be accessed via either MMIO or
port I/O.  This is currently forced to be a compile-time decision.
For example: we currently access a 16550 UART via port I/O on x86 and
via MMIO on any other platform.

PCI UARTs with MMIO BARs do exist but are not currently supported in
an x86 build of iPXE.  Some AWS EC2 systems (observed on a c6i.metal
instance in eu-west-2) provide only a PCI MMIO UART, and it is
therefore currently impossible to get serial output from iPXE on these
instance types.

Add ioread8(), ioread16(), etc accessors that will select between MMIO
and port I/O at the point of use.  For non-x86 platforms where we
currently have no port I/O support, these simply become wrappers
around the corresponding readb(), readw(), etc MMIO accessors.  On
x86, we use the fairly well-known trick of treating any 16-bit address
(below 64kB) as a port I/O address.

This trick works even in the i386 BIOS build of iPXE (where virtual
addresses are offset from physical addresses by a runtime constant),
since the first 64kB of the virtual address space will correspond to
the iPXE binary itself (along with its uninitialised-data space), and
so must be RAM rather than a valid MMIO address range.

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