Documentation: dt-bindings: Document deprecation of "vmmc_aux" and using "vqmmc"

Document deprecation of "vmmc_aux" for io regulator and use of generic
mmc binding "vqmmc" in omap-hsmmc.

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