spi: aspeed: Add support for direct mapping

Use direct mapping to read the flash device contents. This operation
mode is called "Command mode" on Aspeed SoC SMC controllers. It uses a
Control Register for the settings to apply when a memory operation is
performed on the flash device mapping window.

If the window is not big enough, fall back to the "User mode" to
perform the read.

Direct mapping for writes will come later when validated.

Reviewed-by: Joel Stanley <[email protected]>
Tested-by: Joel Stanley <[email protected]>
Tested-by: Tao Ren <[email protected]>
Tested-by: Jae Hyun Yoo <[email protected]>
Signed-off-by: Cédric Le Goater <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mark Brown <[email protected]>

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