ARM: dts: at91: sama5d2_icp: Set sst26vf064b SPI NOR flash at its maximum frequency

sama5d2_icp populates an sst26vf064b SPI NOR flash. Its maximum operating
frequency for 2.7-3.6V is 104 MHz. As the flash is operated at 3.3V,
increase its maximum supported frequency to 104MHz. The increasing of the
spi-max-frequency value requires the setting of the
"CE# Not Active Hold Time", thus set the spi-cs-setup-ns to a value of 7.

The sst26vf064b datasheet specifies just a minimum value for the
"CE# Not Active Hold Time" and it advertises it to 5 ns. There's no
maximum time specified. I determined experimentally that 5 ns for the
spi-cs-setup-ns is not enough when the flash is operated close to its
maximum frequency and tests showed that 7 ns is just fine, so set the
spi-cs-setup-ns dt property to 7.

With the increase of frequency the reads are now faster with ~37%.

Signed-off-by: Tudor Ambarus <[email protected]>
Tested-by: Nicolas Ferre <[email protected]> # on sama5d2 ICP
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Nicolas Ferre <[email protected]>

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