powerpc/44x: Remove STDBINUTILS kconfig option

STDBINUTILS is just a toggle to allow 256k page size to appear in the
possible page sizes list for the 44x.

Make 256k page size option appear all the time with an explicit
warning about binutils, and remove this unnecessary STDBINUTILS
config option.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Incorporate help text changes from David Laight]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f9981e819009aa121a998dc483052ec76f78f991.1611128938.git.christophe.leroy@csgroup.eu