spi: wpcm-fiu: Add direct map support

Besides software controlled SPI transfers (UMA, "user mode access"), FIU
also supports a 16 MiB mapping window per attached flash chip.

This patch implements direct mapped read access, to speed up flash reads.

Without direct mapping:

	# time dd if=/dev/mtd0ro of=dump bs=1M
	16+0 records in
	16+0 records out
	real    1m 47.74s
	user    0m 0.00s
	sys     1m 47.75s

With direct mapping:

	# time dd if=/dev/mtd0ro of=dump bs=1M
	16+0 records in
	16+0 records out
	real    0m 30.81s
	user    0m 0.00s
	sys     0m 30.81s

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20221124191400.287918-4-j.neuschaefer@gmx.net


Signed-off-by: Mark Brown <broonie@kernel.org>