mmc: core: Allow hosts to specify non-support for SD commands

There are host drivers which needs to valdiate for non-supported SD
commands and returnn error code for such requests.

To improve and simplify the behaviour, let's invent MMC_CAP2_NO_SD
which these host drivers can set to tell the mmc core to skip sending SD
commands during card initialization.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>