spl: mmc: Try to clean up raw-mode options

Make the raw-mode options depend on SPL_SYS_MMCSD_RAW_MODE in a more
direct way. This makes it easier to understand the options with
'make menuconfig'.

There are three different ways of specifying the offset:

- sector offset
- partition number
- partition type

So make these a choice, so it is more obvious what is going on.

Update existing boards to enable SPL_SYS_MMCSD_RAW_MODE where needed.

Reviewed-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Simon Glass <sjg@chromium.org>