mmc: sdhci: add support for configurable request size

Some platform drivers support more than 128 adma descriptors
and allows more than 512KB maximum request size per transfer.
Add a callback to get maximum supported adma descriptors from
platform driver instead of limiting host capabilities to
128 descriptors.

Change-Id: I0ac0ffbd0e792a76931e21b321b39c35195ac8d6
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>