dt-bindings: sram: Document reg-io-width property

Some SRAMs need to be accessed with a specific access width, define
the 'reg-io-width' property specifying such access sizes.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Message-Id: <20240827182450.3608307-2-florian.fainelli@broadcom.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>