ARM: dts: imx7d-flex-concentrator: fix pcf2127 reset

RTC pcf2127 device driver has changed default behaviour of the watchdog
feature in v5.11-rc1. Now you need to explicitly enable it with a
device tree property, "reset-source", when used in the board design.

Fixes: 71ac1345

 ("rtc: pcf2127: only use watchdog when explicitly available")
Signed-off-by: Bruno Thomsen <bruno.thomsen@gmail.com>
Cc: Bruno Thomsen <bth@kamstrup.com>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>