mips: Extract HWREna configuration and call from APs

The intent of mips_get_identity is to perform any feature detection and
corresponding global system configuration, but currently it is also
abused to set HWREna.UL on the BSP when available, with APs being left
unconfigured. Extract that part out into its own function that gets
called after mips_get_identity on the BSP, and call it on the APs from
smp_init_secondary.

This is a direct commit to stable/13 as mips no longer exists in main.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D48064