soc: egis: et171: add support for Andes custom CSRs

Egis ET171 implements Andes custom CSRs. Enable the following features:

1. Low level initialization of Andes CSRs
2. HWDSP and PowerBrake extensions with context save/restore
3. EXEC.IT extension

Signed-off-by: Jimmy Zheng <jimmyzhe@andestech.com>
8 files changed