dt-bindings: sdhci-omap: Add K2G specific binding

Add binding for the TI's sdhci-omap controller present in K2G.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>