OMAP3: fix warnings when NAND/ONENAND is not used

Fix build warnings by putting specific used variables
under required #ifdefs for removing:
mem.c:227: warning: unused variable 'f_sec'
mem.c:226: warning: unused variable 'f_off'
mem.c:225: warning: unused variable 'size'
mem.c:224: warning: unused variable 'base'
mem.c:222: warning: unused variable 'gpmc_config'

Signed-off-by: Nishanth Menon <nm@ti.com>