remoteproc: stm32: use reset for hold boot

Use the reset function to handle the hold boot bit in RCC
with device tree handle with MCU_HOLD_BOOT identifier.

This generic reset allows to remove the two specific properties:
- st,syscfg-holdboot
- st,syscfg-tz

This patch prepares alignment with kernel device tree.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Fabien DESSENNE <fabien.dessenne@st.com>
Cc: Arnaud POULIQUEN <arnaud.pouliquen@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>