dt-bindings: i2c: st,stm32-i2c: Document wakeup-source property

Document wakeup-source property. This fixes dtbs_check warnings
when building current Linux DTs:

"
arch/arm/boot/dts/stm32mp153c-dhcom-drc02.dtb: i2c@40015000: Unevaluated properties are not allowed ('wakeup-source' was unexpected)
"

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>