Use 115200 bps by default for serial communication

9600 was a standard baud rate decades ago, but 115200 is now more common
so choose defaults that are useful to the largest number of users.

Note that boot0sio does not support rates above 9600 so it remains
unchanged.

Reviewed by: bz, imp, manu
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36295