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/rcar3_salvator-x_defconfig b/configs/rcar3_salvator-x_defconfig
index d423ad7..bb2101e 100644
--- a/configs/rcar3_salvator-x_defconfig
+++ b/configs/rcar3_salvator-x_defconfig
@@ -87,6 +87,7 @@
 CONFIG_SYS_FLASH_PROTECTION=y
 CONFIG_SYS_FLASH_CFI=y
 CONFIG_RENESAS_RPC_HF=y
+CONFIG_SYS_MAX_FLASH_SECT=256
 CONFIG_SYS_MAX_FLASH_BANKS_DETECT=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH_SPANSION=y