mmc: core: Add support for HS400

HS400 is a new bus speed mode introduced in eMMC5.0 specification. This
patch adds the capability in mmc core to switch to HS400 mode when both
the host and the card supports it.

Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
Signed-off-by: flar2 <asegaert@gmail.com>