mmc, sh: Remove sh_mmcif_boot_slurp()

As the only caller of sh_mmcif_boot_do_read() is
sh_mmcif_boot_slurp() the configuration portion of
sh_mmcif_boot_slurp() can be merged into sh_mmcif_boot_do_read().

Once this is done sh_mmcif_boot_slurp() is only a call
to sh_mmcif_boot_do_read() with platform specific information -
the offset that images are stored on MMC. So make the
sh_mmcif_boot_do_read() call directly from platform code
and remove sh_mmcif_boot_slurp() altogether.

Cc: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>