power: spl: add SPL_DM_REGULATOR_FIXED in Kconfig

The Makefile already tests for SPL_DM_REGULATOR_FIXED, but Kconfig
does not provide it.  This adds SPL_DM_REGULATOR_FIXED to Kconfig.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>