board: stm32mp1: remove test on CONFIG_DM_REGULATOR

The tests on CONFIG_DM_REGULATOR, added to avoid compilation issues, can
now be removed, they are no more needed since the commit 16cc5ad0
("power: regulator: add dummy helper").

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>