mmc: sdhci: Let a vendor driver supply and update ADMA descriptor size

Let a vendor driver supply the maximum descriptor size that it
can operate on. ADMA descriptor table would be allocated using this
supplied size.
If any SD Host controller is of version prior to v4.10 spec
but supports 16byte descriptor, this change allows them to supply
correct descriptor size for ADMA table allocation.

Also let a vendor driver update the descriptor size by overriding
sdhc_host->desc_size if it has to operates on a different descriptor
sizes in different conditions.

Suggested-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/1579531122-28341-1-git-send-email-vbadigan@codeaurora.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>