reset: stm32: Add support of MCU HOLD BOOT

Handle the register RCC_MP_GCR without SET/CLR registers
but with a direct access to bit BOOT_MCU:
- deassert => set the bit: The MCU will not be in HOLD_BOOT
- assert => clear the bit: The MCU will be set in HOLD_BOOT

With this patch the RCC driver handles the MCU_HOLD_BOOT_R value
added in binding stm32mp1-resets.h

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