mtd: maps: add INT0800 firmware-flash map driver

Add a read-only mapping driver that binds the ACPI INT0800 "Intel
82802 firmware hub" device and exposes the system firmware flash as an
MTD ROM device.

On x86 machines the boot flash is decoded into the physical address
space below 4 GB, so a plain ioremap() of the declared resource window
is sufficient for reads - no SPI or LPC controller access is needed.
The window may be larger than the real flash; undecoded holes read as
0xff.

This gives userspace a clean, safe way to read firmware flash contents
without flashrom or relaxed /dev/mem access - for firmware analysis,
and for extracting option ROMs stored inside EFI firmware volumes
(e.g. the NVIDIA VBIOS on EFI-booted Apple machines, which can then be
fed to nouveau via nouveau.config=NvBios=). There is deliberately no
write or erase support.

Tested on a MacBookPro4,1 (ICH8M): /dev/mtd0 reads are byte-identical
to a flashrom dump of the 2 MiB SST25VF016B, except for live NVRAM
variable-store regions.

Signed-off-by: Stephen Bancroft <stevereaver@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

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