sh: fix build regression with CONFIG_OF && !CONFIG_OF_FLATTREE

Such a configuration could only be selected by manually selecting
CONFIG_OF; SH_DEVICE_TREE selects both. The affected code is using the
flat DTB at boot time and thus rightfully should depend on
OF_FLATTREE, not just OF.

Signed-off-by: Rich Felker <dalias@libc.org>