ARM: dts: at91/trivial: Fix USART1 definition for at91sam9g45

Fix a typo. No functional change made by this patch.

Signed-off-by: Jay Foster <jayfoster@ieee.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>