mpc83xx: fix undefined reference to `flush_cache' error in simpc8313 build

mpc83xx: fix undefined reference to `flush_cache' error in simpc8313 build

extend commit c70564e6b1bd08f3230182392238907f3531a87e
"NAND: Fix cache and memory inconsistency issue" to add the cache.o dependency
to the simpc8313 build and fix this:

...Large Page NAND...Configuring for SIMPC8313 board...
nand_boot_fsl_elbc.o: In function `nand_boot':
nand_spl/board/sheldon/simpc8313/nand_boot_fsl_elbc.c:150: undefined reference to `flush_cache'
make[1]: *** [/home/r1aaha/git/u-boot-mpc83xx/nand_spl/u-boot-spl] Error 1
make: *** [nand_spl] Error 2

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>