spi: imx: support word delay

Implement support for the word delay feature of i.MX51 (and onwards) via
the ECSPI interface.

Convert the requested delay to SPI cycles and account for an extra
inter-word delay inserted by the controller in addition to the requested
number of cycles, which was observed when testing this patch.

Disable dynamic burst when word delay is set. As the configurable delay
period in the controller is inserted after bursts, the burst length must
equal the word length.

Account for word delay in the transfer time estimation for
polling_limit_us.

Signed-off-by: Jonas Rebmann <[email protected]>
Reviewed-by: Frank Li <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Mark Brown <[email protected]>

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