spi: dt-bindings: dw-apb-ssi: update spi-{r,t}x-bus-width

spi: dt-bindings: dw-apb-ssi: update spi-{r,t}x-bus-width

Most users of dw-apb-ssi use spi-{r,t}x-bus-width of 1, however the
Canaan k210 is wired up for a width of 4.
Quoting Serge:
The modern DW APB SSI controllers of v.4.* and newer also support the
enhanced SPI Modes too (Dual, Quad and Octal). Since the IP-core
version is auto-detected at run-time there is no way to create a
DT-schema correctly constraining the Rx/Tx SPI bus widths.
/endquote

As such, drop the restriction on only supporting a bus width of 1.

Link: https://lore.kernel.org/all/20220620205654.g7fyipwytbww5757@mobilestation/
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: Niklas Cassel <niklas.cassel@wdc.com>
Link: https://lore.kernel.org/r/20220629184343.3438856-5-mail@conchuod.ie
Signed-off-by: Mark Brown <broonie@kernel.org>