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-evaluation_defconfig b/configs/stm32f429-evaluation_defconfig
index e2c41b0..98c0507 100644
--- a/configs/stm32f429-evaluation_defconfig
+++ b/configs/stm32f429-evaluation_defconfig
@@ -31,4 +31,5 @@
 CONFIG_ARM_PL180_MMCI=y
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_STM32_FLASH=y
+CONFIG_SYS_MAX_FLASH_SECT=12
 CONFIG_SYS_MAX_FLASH_BANKS=2