blk: blkmap: add ramdisk creation utility function

User needs to call several functions to create the ramdisk
with blkmap.
This adds the utility function to create blkmap device and
mount the ramdisk.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>