dts: arm: st: h7: fix stm32h7 separate cores

Currently, only the stm32h747 soc is supported in the h7 foler.
The h7 series comes with both single core and dual core products.
This change moves C-M4 core out of stm32h7.dtsi so that it can be
included by single core STM32H7 soc description.

Signed-off-by: Moonkwun Jung <mkainyh@gmail.com>