mmc: sdhci_am654: Constify struct regmap_config

`sdhci_am654_regmap_config` is not modified and can be declared as const
to move its data to a read-only section.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://lore.kernel.org/r/20240705-sdhci_am654-const-regmap_config-v1-1-c166a8d48a66@gmail.com


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