Add MMC framework

Add MMC framework

This change is largely based on existing eMMC framework by Haojian Zhuang
(@hzhuang1).

The MMC framework supports both eMMC and SD card devices. It was
written as a new framework since breaking few eMMC framework APIs.

At card probe and after the reset to idle command (CMD0), a Send
Interface Condition Command is sent (CMD8) to distinguish between
eMMC and SD card devices. eMMC devices go through the same
sequence as in the former eMMC framework. Else the framework
uses commands dedicated to SD-cards for init or frequency switch.

A structure is created to share info with the driver. It stores:
- the MMC type (eMMC, SD or SD HC)
- the device size
- the max frequency supported by the device
- the block size: 512 for eMMC and SD-HC and read from CSD
 structure for older SD-cards

Restriction to align buffers on block size has been removed.
Cache maintenance was removed and is expected to be done in the platform
or device driver.

The MMC framework includes some MISRA compliance coding style
maybe not yet ported in the existing eMMC framework.

Fixes ARM-software/tf-issues#597

Signed-off-by: Yann Gautier <yann.gautier@st.com>