arch: arm: cortex_m: Add API for SCB save and restore

Add new API to save and restore SCB context. This is typically useful when
entering and exiting suspend-to-RAM low-power modes.

The scb_context_t and the backup/restore functions are designed to only
handle SCB registers that are:
- Mutable: Their values can be changed by software.
- Configurable: They control system behavior or features.
- Stateful: Their values represent a specific configuration that an
            application might want to preserve and restore.

Registers excluded from backup/restore are:
1. CPU/feature identification registers
	Motivation: These registers are fixed in hardware and read-only.
2. ICSR (Interrupt Control and State Register)
	Motivation: Most bits of ICSR bits are read-only or write-only
	and represent volatile system state. STTNS is the only read-write
	field and could be considered part of the system state, but it is
	only present on certain ARMv8-M CPUs, and Zephyr does not use it.
3. CFSR (Configurable Fault Status Register)
   HFSR (HardFault Status Register)
   DFSR (Debug Fault Status Register)
   AFSR (Auxiliary Fault Status Register)
   MMFAR (MemManage Fault Address Register)
   BFAR (BusFault Address Register)
	Motivation: These registers are read/write-one-to-clear and
	contain only fault-related information (which is volatile).

Co-authored-by: Mathieu Choplain <mathieu.choplain@st.com>

Signed-off-by: Michele Sardo <msmttchr@gmail.com>
Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>