mmc: sdhci-brcmstb: Add ability to use HS400ES transfer mode

The latest eMMC JEDEC specification version 5.1 added a new
transfer mode, HS400 with enhanced strobe (HS400ES). This mode
will be selected if both the host controller and eMMC device
support it. The latest Arasan 5.1 controller in the 7216a0
supports this mode. The "Host Controller Specification" has
not been updated so the controller register bit used to enable
this mode is not specified and varies the with controller vendor.
The Linux SDHCI driver supplies a callback for enabling HS400ES
mode and that callback will be used to supply a routine that
will set the proper bit in the Arasan Vendor register.

Signed-off-by: Al Cooper <alcooperx@gmail.com>
Link: https://lore.kernel.org/r/20200113210706.11972-3-alcooperx@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>