Convert imx6_esdhc_start_image() into a generic esdhc_start_image() by
making accept already filled "struct esdhc" as well as use
esdhc_read_blocks().

With that change, create new imx6_esdhc_start_image() whose sole task
is to properly fill a struct esdhc with appropriate offset and to pass
it on to esdhc_start_image().

Both changes are made with a goal of simplifying adding support of new
SoC in mind (see following commits adding support for i.MX8 as example).

NOTE: This commit does not re-implement imx6_esdhc_load_image(),
instead opting for dropping it, due to lack of any users in the
codebase.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>