dts: arm: st: stm32c0: add stm32c051

Add dts for stm32c051.
STM32C051 is just an STM32c071 w/o USB and different memory configuration.
Therefore STM32C091 can directly include STM32C051 and no longer needs
to delete the usb nodes.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>