tcl/target: Add RCPU support for Spacemit K1

tcl/target: Add RCPU support for Spacemit K1

Add support for the Real-Time CPU (RCPU) of K1, which is a 32-bit RISC-V
N308 High-Efficiency Processor Core designed by Nuclei System Technology
Co. Ltd.

The JTAG interface can be configured to connect to either X60s or RCPU
processors. To enable JTAG for RCPU, set TARGET to "rcpu".

For example:
openocd -c "set TARGET rcpu" -f interface/cmsis-dap.cfg \
        -f target/spacemit-k1.cfg

Change-Id: I9cd62fac332137afac17efa52702818de8f0b6f5
Signed-off-by: Junhui Liu <junhui.liu@pigmoral.tech>
Reviewed-on: https://review.openocd.org/c/openocd/+/8821
Reviewed-by: liangzhen <zhen.liang@spacemit.com>
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>