arm64: dts: imx8mm-beacon: Fix RTC capacitive load

[ Upstream commit 2e98d456 ]

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: 593816fa ("arm64: dts: imx: Add Beacon i.MX8m-Mini development kit")
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>