selftests/nolibc: use file driver for QEMU serial

For the test implementation of the SuperH architecture a second serial
serial port needs to be used. Unfortunately the currently used 'stdio'
driver does not support multiple serial ports at the same time.

Switch to the 'file' driver which does support multiple ports and is
sufficient for the nolibc-test usecase.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/20250623-nolibc-sh-v2-2-0f5b4b303025@weissschuh.net