dt-bindings: mmc: Add Broadcom optional sdio_freq clock

The 72116B0 has improved SDIO controllers that allow the max clock
rate to be increased from a max of 100MHz to a max of 150MHz.
Optional "sdio_freq" clock is used to drive the bus clock if present
optional property "clock-frequency" specifies a base clock frequency
in Hz that overrides the base clock frequency in the CAPS registers.

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220520183108.47358-2-kdasu.kdev@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>