Added sbytemask field to textra32 and textra64 (#588)

* Added sbytemask field to textra32 and textra64

sbytemask field in textra32 and textra64 provides byte-granular comparison of the svalue trigger value to the scontext CSR

* Update xml/hwbp_registers.xml

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>

* Added comments about sbytemask

Byte masking of svalue comparison only applies when sselect==1.

* Update hwbp_registers.xml

* Update hwbp_registers.xml

* Update hwbp_registers.xml

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>