mmc: tmio: add hook for custom busy_wait calculation

Newer SDHI variants can 'wait while busy' longer than the generic TMIO.
Provide a hook to get the maximum cycle count to wait for. If the hook
is not populated, fall back to a generic version which works well with
all older TMIO/SDHI variants.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20201125213001.15003-3-wsa+renesas@sang-engineering.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>