common/fdt_support.c: Fix compile warnings

Commit a6bd9e83 "FDT: Add fixup support for multiple banks of memory"
removed code but forgot to remove the variables used by it, resulting
in warnings:

fdt_support.c: In function 'fdt_fixup_memory_banks':
fdt_support.c:399: warning: unused variable 'sizecell'
fdt_support.c:399: warning: unused variable 'addrcell'

Remove the declarations, too.

Signed-off-by: Wolfgang Denk <wd@denx.de>