libfdt: Change names of sparse helper macros

The default libfdt_env.h (for POSIXish userland builds) supports sparse
checking.  It has a couple of helper macros, __force and __bitwise which
expand the relevant sparse attributes to enable checking for incorrect
or missing endian conversions.

Those are bad names: for one, leading underscores are supposed to be
reserved for the system libraries, and worse, some systems (including
RHEL7) do define those names already.

So change them to FDT_FORCE and FDT_BITWISE which are far less likely to
have collisions.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>