drivers: ppp: Move to DTS for uart device

Move from using Kconfig NET_PPP_UART_NAME to a devicetree chosen
property ("zephyr,ppp-uart").  This is similar to a number of other
functions like "zephyr,shell-uart" or "zephyr,bt-uart".

As part of this we rework the init code a little to use
DEVICE_DT_GET for the modem gsm-ppp case.

Signed-off-by: Kumar Gala <galak@kernel.org>