ARM: dts: stm32: Update Cortex-M4 reset declarations on stm32mp15

Since the introduction of the SCMI for the management
of the MCU hold boot in OP-TEE, management of the hold boot by SMC call
is deprecated.
- Clean the st,syscfg-tz  which allows to determine if the trust
zone is enable.
- Add reset-names properties to be able to differentiate the MCU reset and
  the MCU HOLD BOOT.

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