mmc: add bkops-enable command

Add new command that provides possibility to enable the
background operations handshake functionality
(BKOPS_EN, EXT_CSD byte [163]) on eMMC devices.

This is an optional feature of eMMCs, the setting is write-once.
The command must be explicitly taken into use with
CONFIG_CMD_BKOPS_ENABLE.

Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>