mmc: jz4740: Introduce devicetree probe

Add support to probe the device via devicetree, which
will be used to support other SoCs such as the JZ4780.

Based on commits from the CI20 repo, by Paul Cercueil
and Alex Smith. Binding document based on work by
Zubair Lutfullah Kakakhel.

Tested-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.co.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>