mtd: rawnand: gpmi: validate controller clock rate

What to do when the real rate of the gpmi clock is not equal to the
required one? The solutions proposed in [1] did not lead to a conclusion
on how to validate the clock rate, so, inspired by the document [2], I
consider the rate correct only if not lower or equal to the rate of the
previous edo mode. In fact, in chapter 4.16.2 (NV-DDR) of the document [2],
it is written that "If the host selects timing mode n, then its clock
period shall be faster than the clock period of timing mode n-1 and
slower than or equal to the clock period of timing mode n.". I thought
that it could therefore also be used in this case, without therefore
having to define the valid rate ranges empirically.

For example, suppose that gpmi_nfc_compute_timings() is called to set
edo mode 5 (100MHz) but the rate returned by clk_round_rate() is 80MHz
(edo mode 4). In this case gpmi_nfc_compute_timings() will return error,
and will be called again to set edo mode 4, which this time will be
successful.

[1] https://lore.kernel.org/r/[email protected]
[2] http://www.onfi.org/-/media/client/onfi/specs/onfi_3_0_gold.pdf?la=en

Co-developed-by: Michael Trimarchi <[email protected]>
Signed-off-by: Michael Trimarchi <[email protected]>
Signed-off-by: Dario Binacchi <[email protected]>
Tested-by: Sascha Hauer <[email protected]>
Reviewed-by: Sascha Hauer <[email protected]>
Signed-off-by: Miquel Raynal <[email protected]>
Link: https://lore.kernel.org/linux-mtd/[email protected]

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