serial: Rename .init() and .uninit() in serial_device

Rename .init() to .start() and .uninit() to .stop() in struct
serial_device. This allows aligning struct serial_device with
closer to struct stdio_dev. The real goal here is to allow
these two structures to converge together and eventually make
one to be a superset of the other.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Tom Rini <trini@ti.com>