boot: zephyr: socs: stm32h7s3xx: Add support for ext_flash_app variant

[TD-JBL]

Based on PR #2510

On the same example as stm32h7s7xx, introduce ext_flash_app variant for stm32h7s3xx in mcuboot.

This allows chainloading applications in ext flash from a mcuboot application running in internal flash.

Similarly, BOOT_MAX_IMG_SECTORS_AUTO is no longer needed to be disabled after including the 'soc-nv-flash' binding in the 'st,stm32-xspi-nor' binding, which provides 'erase-block-size' and 'write-block-size' to external Flash DT node.

Remove the no longer required Kconfig fragment for nucleo_h7s3l8.