Fix compiler warning in fdt_support.c

Fix compiler warning

fdt_support.c: In function 'of_bus_default_count_cells':
fdt_support.c:957: warning: passing argument 1 of '__swab32p' discards qualifiers from pointer target type
fdt_support.c:965: warning: passing argument 1 of '__swab32p' discards qualifiers from pointer target type

be32_to_cpup() expects an 'u32 *' while prop is 'const u32 *'.

Signed-off-by: Dirk Behme <dirk.behme@googlemail.com>