spl: Kconfig: de-dup SPL_DM_GPIO definition

Two nearly concurrent commits (d4d65e11 and bcee8d67) added a
SPL_DM_GPIO symbol. Resolve the duplication in favor of the version
in drivers/gpio/Kconfig.

Signed-off-by: Joel Johnson <mrjoel@lixil.net>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>