ARM: dts: am335x-myirtech: Add an external clock oscillator for CPU RTC

The CPU RTC has an external crystal, so to keep time, let's define
it for devicetree.

Signed-off-by: Alexander Shiyan <eagle.alexander923@gmail.com>
Message-Id: <20220427081914.6773-1-eagle.alexander923@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>