mmc: enable Enhance Strobe for HS400

Enhance Strobe is defined in v5.1 eMMC spec. This commit
is to implement it.

Normal Strobe signal for HS400 is only provided during
Data Out and CRC Response. While Enhance Strobe is enabled,
Strobe signal is provided during Data Out, CRC Response and
CMD Response.

While enabling Enhance Strobe, the initialization of HS400
does not need enabling HS200 and executing tuning anymore.
This simplifies the HS400 initialization process much.

Per spec, there is a STROBE_SUPPORT added in EXT_CSD register
to indicate that card supports Enhance Strobe or not. If it is
supported, host can enable this feature by enabling the most
significant bit of BUS_WIDTH before set HS_TIMING to HS400.

Change-Id: I1003352cb31dfaec01fde352da7b879a13c94b3f
Signed-off-by: Yi Sun <yi.y.sun@intel.com>
[venkatg@codeaurora.org: Fix minor conflicts & compilation failure]
Patch-mainline: linux-mmc @ 06/04/15, 19:50
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>