arm64: Fix the sign in ID registers

Some ID registers use 0b0000 to indicate a feature is enabled, and
0b1111 to indicate it's not. In these cases we need to tell the kernel
to treat the value as a signed field.

Set the signed flag in fields we decode that should be signed.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D51369