ARM: dts: imx6: phytec: fix RTC interrupt level

RTC interrupt level should be set to "LOW". This was revealed by the
introduction of commit:

  f181987e ("rtc: m41t80: use IRQ flags obtained from fwnode")

which changed the way IRQ type is obtained.

Signed-off-by: Andrej Picej <andrej.picej@norik.com>
Reviewed-by: Stefan Riedmüller <s.riedmueller@phytec.de>
Fixes: 800d5951 ("ARM: dts: imx6: Add initial support for phyBOARD-Mira")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>