dt-bindings: sdhci-omap: Add am335x and am437x specific bindings

Add binding for the TI's sdhci-omap controller present in am335x and
am437x devices.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200116105154.7685-9-faiz_abbas@ti.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>