termios: add more speeds

A lot of small arm64 gadgets are using 1500000 as console speed.
While cu can perfectly deal with this some 3rd party software, e.g.,
comms/conserver-con add speeds based on B<n> being defined.
Having it defined here simplifies enhancing other software.

Obtained-from: NetBSD sys/sys/termios.h 1.36
MFC-after: 2 weeks
Reviewed-by: philip (,okayed by imp)
Differential Revision: https://reviews.freebsd.org/D29209