arm64: dts: imx8mm-phyboard-polis: add rtc aux-voltage-chargeable

phyboard-polis has a chargable capacitor populated, rtc supports
charging it. Add property indicating this.

Signed-off-by: Yannic Moog <y.moog@phytec.de>
Reviewed-by: Teresa Remmet <t.remmet@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>