stm32mp: stpmic1: remove the debug unit request by debugger

Depending on backup register value, U-Boot SPL maintains the debug unit
powered-on for debugging purpose; only BUCK1 is required for powering
the debug unit, so revert the setting for all the other power lanes,
except BUCK3 that has to be always on.

To be functional this patch requires a modification in the debugger
,openocd for example, to update the STM32MP15 backup register when it is
required to debug SPL after reset. After deeper analysis this behavior
will be never supported in tools so the associated code, will be never
used and the associated code can be removed.

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