ARM: dts: stm32: fix m4_rproc references to use SCMI for stm32mp15

ARM: dts: stm32: fix m4_rproc references to use SCMI for stm32mp15

Fixes stm32mp15*-scmi DTS files introduced in [1]:
This patch fixes the node which uses the MCU reset and adds the
missing HOLD_BOOT which is also handled by the SCMI reset service.

This change cannot be applied as a fix on commit [1], the management
of the hold boot impacts also the stm32_rproc driver.

[1] 'commit 5b7e58313a77 ("ARM: dts: stm32: Add SCMI version of STM32 boards (DK1/DK2/ED1/EV1)")'

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