ARM: dts: stm32: Fix spi4 pins in stm32mp15-pinctrl

Move spi4_pins_a nodes from pinctrl_z to pinctrl as the associated pins
are not in BANK Z.

Fixes: 498a7014 ("ARM: dts: stm32: Add missing pinctrl entries for STM32MP15")

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>