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/stm32f746-disco_spl_defconfig b/configs/stm32f746-disco_spl_defconfig
index 927d28d..0368ece 100644
--- a/configs/stm32f746-disco_spl_defconfig
+++ b/configs/stm32f746-disco_spl_defconfig
@@ -63,6 +63,7 @@
 CONFIG_DM_MTD=y
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_STM32_FLASH=y
+CONFIG_SYS_MAX_FLASH_SECT=8
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH_MACRONIX=y
 CONFIG_SPI_FLASH_STMICRO=y