nand/denali: Document CONFIG symbols

The patch "nand/denali: Adding Denali NAND driver support"
introduced two config symbols without documenting them.
Signed-off-by: NScott Wood <scottwood@freescale.com>
Cc: Chin Liang See <clsee@altera.com>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>

doc/README.nand

@@ -215,6 +215,12 @@ Configuration Options:
 
 Platform specific options
 =========================
+   CONFIG_NAND_DENALI
+	Enables the denali.c driver.
+
+   CONFIG_SYS_NAND_DENALI_64BIT
+	Indicates that the Denali NAND controller is the 64-bit variant.
+
    CONFIG_NAND_OMAP_GPMC
 	Enables omap_gpmc.c driver for OMAPx and AMxxxx platforms.
 	GPMC controller is used for parallel NAND flash devices, and can