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

sys/sys/_termios.h

@@ -208,7 +208,15 @@
 #define	B115200	115200
 #define	B230400	230400
 #define	B460800	460800
+#define B500000 500000
 #define	B921600	921600
+#define B1000000	1000000U
+#define B1500000	1500000U
+#define B2000000	2000000U
+#define B2500000	2500000U
+#define B3000000	3000000U
+#define B3500000	3500000U
+#define B4000000	4000000U
 #define	EXTA	19200
 #define	EXTB	38400
 #endif