Convert CONFIG_SYS_MAX_FLASH_SECT to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MAX_FLASH_SECT

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/stm32f469-discovery_defconfig b/configs/stm32f469-discovery_defconfig
index c7dbc69..c27f760 100644
--- a/configs/stm32f469-discovery_defconfig
+++ b/configs/stm32f469-discovery_defconfig
@@ -33,6 +33,7 @@
 CONFIG_DM_MTD=y
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_STM32_FLASH=y
+CONFIG_SYS_MAX_FLASH_SECT=12
 CONFIG_SYS_MAX_FLASH_BANKS=2
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y