spi: cadence-quadspi: Add minimum operable clock rate warning to baudrate divisor calculation

This Cadence QSPI IP has a 4-bit clock divisor field
for baud rate division.  For example:

0b0000 = /2
0b0001 = /4
0b0010 = /6
...
0b1111 = /32

The maximum divisor is 32
(when div = CQSPI_REG_CONFIG_BAUD_MASK).

If we assume a reference clock of 500MHz and we set
our spi-max-frequency to something low, such as 10 MHz.
The calculated bit field for the divisor ends up being:

DIV_ROUND_UP(500000000/(2*10000000))-1 = 25

25 is 0b11001... which truncates to a divisor field of 0b1001 (or /20).

This is higher than our anticipated max-frequency of 10MHz
(500MHz/20 = 25 MHz).  Instead, let's make sure we're always using
the maximum divisor (/32) in this case and give the user a warning about
the rate adjustment.

Signed-off-by: Nathan Barrett-Morrison <nathan.morrison@timesys.com>
Link: https://lore.kernel.org/r/20221128164147.158441-1-nathan.morrison@timesys.com


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