doc: Update parallel NOR flash related information in README.falcon

This commit updates the doc/README.falcon regarding Falcon boot on
NOR flash memories.

This code is used by MCCMON6 board - so for more details please refer to
configs/mccmon6_nor_defconfig.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
diff --git a/doc/README.falcon b/doc/README.falcon
index 9a7f0bc..713d706 100644
--- a/doc/README.falcon
+++ b/doc/README.falcon
@@ -67,6 +67,8 @@
 
 CONFIG_CMD_SPL_NAND_OFS	Offset in NAND where the parameters area was saved.
 
+CONFIG_CMD_SPL_NOR_OFS	Offset in NOR where the parameters area was saved.
+
 CONFIG_CMD_SPL_WRITE_SIZE 	Size of the parameters area to be copied
 
 CONFIG_SPL_OS_BOOT	Activate Falcon Mode.