cmd: mmc: Expand bkops handling

Add more capable "bkops" command which allows enabling and disabling both
manual and automatic bkops. The existing 'mmc bkops-enable' subcommand is
poorly named to cover all the possibilities, hence the new-ish subcommand.
Note that both commands are wrappers around the same common code.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>