mmc: fsl_esdhc: Implement card-detect hook.

This card-detect hook probably doesn't work. Perhaps somebody with more
knowledge about the hardware can comment on this. I think that perhaps
even the complete code from esdhc_init() could go into the getcd()
function instead or mmc_getcd() needs to be called at some later time
after mmc_init(), which, however, would require many other drivers to
change.

In addition to implementing the hook, this patch also removes the call
to the board_mmc_getcd() function which is now called from the MMC
framework and is no longer required here.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Tested-by: Jason Liu <jason.hui@linaro.org>