arm64: dts: imx8mn-beacon: Fix RTC capacitive load

Although not noticeable when used every day, the RTC appears to drift when
left to sit over time.  This is due to the capacitive load not being
properly set. Fix RTC drift by correcting the capacitive load setting
from 7000 to 12500, which matches the actual hardware configuration.

Fixes: 36ca3c8c ("arm64: dts: imx: Add Beacon i.MX8M Nano development kit")
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>