riscv: dts: sophgo: cv18xx: Add RTCSYS device node

riscv: dts: sophgo: cv18xx: Add RTCSYS device node

Add the RTCSYS MFD node: in Cvitek CV18xx and its successors RTC Subsystem
is quite advanced and provides SoC power management functions as well.

The SoC family also contains DW8051 block (Intel 8051 compatible CPU core)
and an associated SRAM. The corresponding control registers are mapped into
RTCSYS address space as well.

Link: https://github.com/sophgo/sophgo-doc/tree/main/SG200X/TRM
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Reviewed-by: Inochi Amaoto <inochiama@gmail.com>
Link: https://lore.kernel.org/r/20250513203128.620731-1-alexander.sverdlin@gmail.com
Signed-off-by: Inochi Amaoto <inochiama@gmail.com>
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
Signed-off-by: Chen Wang <wangchen20@iscas.ac.cn>