debug registers: Add missing debug version entry for FEAT_Debugv8p8

debug registers: Add missing debug version entry for FEAT_Debugv8p8

The Arm Architecture Reference Manual defines debug version 0b1010 for
FEAT_Debugv8p8. This is used to identify valid hardware debug registers.

gdb currently only knows about versions up to FEAT_Debugv8p4. This patch
teaches gdb about this new version.

No visible changes should happen as consequence of this patch, but in the
future gdb will be able to identify debug registers in newer hardware.

Regression-tested on aarch64-linux Ubuntu 20.04/22.04.