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/stm32f429-discovery_defconfig b/configs/stm32f429-discovery_defconfig
index fde427a..b347347 100644
--- a/configs/stm32f429-discovery_defconfig
+++ b/configs/stm32f429-discovery_defconfig
@@ -32,4 +32,5 @@
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_STM32_FLASH=y
+CONFIG_SYS_MAX_FLASH_SECT=12
 CONFIG_SYS_MAX_FLASH_BANKS=2