configs: stm32f769-disco: Fix internal flash size

arch-stm32f7/stm32.h file is shared between STM32F746 and STM32F769
MCUs. But STM32F769 embeds 2MB of internal flash instead of 1MB for
STM32F746. The flash layout is quite similar between the 2 SoCs :

	STM32F746 			STM32F769
4 *  32KB sectors 			4 *  32KB sectors
1 * 128KB sector 			1 * 128KB sector
3 * 256KB sectors			7 * 256KB sectors

Update sect_sz_kb[] structure and SYS_MAX_FLASH_SECT accordingly.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Tested-by: Waldemar Brodkorb <wbx@openadk.org>