ARM: dts: stm32: fix stpmic node for stm32mp1 boards

On some STM32 MP15 boards, stpmic node is not correct which generates
warnings running "make dtbs_check W=1" command. Issues are:

-"regulator-active-discharge" is not a boolean but an uint32.
-"regulator-over-current-protection" is not a valid entry for vref_ddr.
-LDO4 has a fixed voltage (3v3) so min/max entries are not allowed.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>