Switch etzpc bus to simple-bus to prevent breakage on non-TFA systems.
This fixes boot of every STM32MP15xx DHSOM device.
Fixes: ad3cdc677dda ("ARM: stm32mp: add ETZPC system bus driver for STM32MP1") Signed-off-by: Marek Vasut <marek.vasut@mailbox.org> Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>