mmc: core: Move erase/trim/discard defines from public core.h to mmc.h

As the public mmc.h header already contains similar defines for other mmc
commands and arguments, let's move those for erase/trim/discard into here
as well.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>