board/ti/omap2420h4/omap2420h4.c: Fix GC 4.6 build warning

Fix:
omap2420h4.c: In function 'dram_init':
omap2420h4.c:196:25: warning: variable 'cpu' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Richard Woodruff <r-woodruff2@ti.com>
Cc: Tom Rini <trini@ti.com>