ARM: OMAP: Enable 8-bit eMMC access for OMAP4/5/DRA7xx

Enable 8-bit host capability for HSMMC2 and/or HSMMC3. CONFIG_HSMMC2_8BIT
(for OMAP4/5/DRA7xx) and/or CONFIG_HSMMC3_8BIT (for DRA7xx only) must be
defined in the board header if an 8-bit eMMC device is connected to the
corresponding port.

Fix the "No status update" error that appeared for eMMC devices by
inserting a 20 us delay between writing arguments and command. This
solution has been proposed by Michael Cashwell <mboards@prograde.net>.

A minor cosmetic fix in a comment as well.

Signed-off-by: Lubomir Popov <lpopov@mm-sol.com>