boot: zephyr: socs: stm32h7s7xx: Add support for ext_flash_app variant

On the same example as stm23h750xx and stm32h573xx, introduce
ext_flash_app variant for stm32h7s7xx 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, whihc provides 'erase-block-size' and 'write-block-size' to
external Flash DT node.

Remove the no longer required Kconfig fragment for stm32h7s78_dk.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>

Author: erwango

boot/zephyr/boards/stm32h7s78_dk.conf

@@ -0,0 +1 @@
+CONFIG_STM32_MEMMAP=y

boot/zephyr/socs/stm32h7s7xx_ext_flash_app.conf

@@ -0,0 +1,7 @@
+/ {
+	chosen {
+		zephyr,flash = &flash0;
+		zephyr,flash-controller = &flash;
+		zephyr,code-partition = &boot_partition;
+	};
+};