mmc: dw_mmc-k3: make array hs_timing_cfg static

The array hs_timing_cfg is local to the source and does not need to
be in global scope, so make it static.

Cleans up sparse warning:
symbol 'hs_timing_cfg' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>