cfi-flash: Add support for mapping flash into guest

At the moment we trap *every* access to the flash memory, even when we
are in array read mode (which just directly copies from the storage
array to the guest).
To improve performance, allow cacheable mappings and to avoid fatal traps
on unsupported instructions (on ARM), export a read-only memslot to the
guest when the flash is in read-array mode. A guest does not need to
trap on read accesses then.
A write command (which always traps) will revoke this mapping if the
read mode changes.

This reduces the number of read traps from more than 800,000 to a few
hundreds when booting into the UEFI shell.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>