arm64: add PMBSR_MSS_{BSC,FSC} status code field

Bits [5:0] of PMBSR_MSS encodes either Buffer Status Code (BSC) or Fault
Status Code (FSC) depending on PMBSR_EC value.

Add PMBSR_MSS_{BSC,FSC} to cover this field.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45172