It is currently not possible to build the all-drivers iPXE binaries
for ARM, since there is no implementation for inb(), outb(), etc.

There is no common standard for accessing I/O space on ARM platforms,
and there are almost no ARM-compatible peripherals that actually
require I/O space accesses.

Provide dummy implementations that behave as though no device is
present (i.e. ignore writes, return all bits high for reads).  This is
sufficient to allow the all-drivers binaries to link, and should cause
drivers to behave as though no I/O space peripherals are present in
the system.

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