Documentation: Remove ZBOOT MMC/SDHI utility and docs

Remove ZBOOT MMC/SDHI Documentation for sh7372 together
wit the vrl4 utility. Without sh7372 and Mackerel support
these files are no longer useful.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>