emmc: support CMD23 ...

Support CMD23. When CMD23 is used, CMD12 could be avoided.

Two scenarios:
1. CMD17 for single block, CMD18 + CMD12 for multiple blocks.
2. CMD23 + CMD18 for both single block and multiple blocks.

The emmc_init() should initialize whether CMD23 is supported
or not.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>

drivers: add emmc stack ...

In a lot of embedded platforms, eMMC device is the only one storage
device. So loading content from eMMC device is required in ATF.

Create the emmc stack that could co-work with IO block driver.
Support to read/write/erase eMMC blocks on both rpmb and normal
user area. Support to change the IO speed and bus width.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>