nand_boot_fsl_nfc.c: make "nfc" a "static const" pointer

With -fPIC enabled, this variable needs an entry in the GOT, which
causes the image size to exceed 2 KiB which is the maximum allowed for
some systems. Making it a "static const" avoids the GOT entry and thus
reduces the image size to < 2 KiB.

Portions of this work were supported by funding from
the CE Linux Forum.

Signed-off-by: Heiko Schocher <hs@denx.de>
Acked-by: Scott Wood <scottwood@freescale.com>