disk: part: refactor generic name creation for DOS and ISO

In both DOS and ISO partition tables the same code to create partition name
like "hda1" was repeated.

Code moved to into a new function part_set_generic_name() in part.c and optimized.
Added recognition of MMC and SD types, name is like "mmcsda1".

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Steve Rae <steve.rae@raedomain.com>
Reviewed-by: Simon Glass <sjg@chromium.org>