spi: spi-mem: allow specifying the maximum frequency on an op

Some SPI NOR operations like the Read SFDP command can only be run at a
specified maximum frequency (50MHz in JESD216D), even though other
commands like read and write can run at higher frequencies.

Allow specifying a maximum frequency of a spi-mem operation so that
controllers can be made aware of any speed restrictions on a command.
Since most controllers can't run at any arbitrary frequency, they are
allowed to run at any frequency below or equal to the maximum. If the
maximum is 0, they can run at their "normal" frequency.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>