rpi: advertise correct default speed of miniuart

Does not implement setting the baud rate yet, but correctly
advertises that we set it to 921600 by default.

kernel/arch/aarch64/rpi_miniuart.c

@@ -128,6 +128,7 @@ static void miniuart_thread(void * arg) {
 	pty->slave->gid = 2; /* dialout group */
 	pty->slave->mask = 0660;
 	pty->_private = arg;
+	pty->tios.c_cflag = CREAD | CS8 | B921600;
 	vfs_mount("/dev/ttyUART1", pty->slave);
 
 	/* Enable interrupts */