mmc: Tinification of the mmc code

Add new configuration option CONFIG_MMC_TINY which strips away all
memory allocation within the MMC code and code for handling multiple
cards. This allows extremely space-constrained SPL code use the MMC
framework.

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