arm: omap2: Fix compiler warning

Fix build warning in arch/arm/mach-omap2/emif-common.c and
arch/arm/mach-omap2/omap4/emif.c with gcc 7.3.0:

  warning: duplicate 'const' declaration specifier [-Wduplicate-decl-specifier]

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>