ARM: dts: stm32: phycore-stm32mp15: qspi: Fix memory map and pinctrl

ARM: dts: stm32: phycore-stm32mp15: qspi: Fix memory map and pinctrl

- Add missing chip select pin group in pinctrl.
- Overwrite the memory map to the Flash device size (16MB) is necessary
to avoid waste of virtual memory that will not be used.
Without this modification, qspi probe fails because of ioremap error.

Signed-off-by: Christophe Parant <c.parant@phytec.fr>
Link: https://lore.kernel.org/r/20251210101611.27008-7-c.parant@phytec.fr
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>