[MTD] Nuke IVR leftovers

Support for the ITE8172 based boards was deleted a while ago so this is
dead code.

The Kconfig dependency on MIPS was wrong anyway, MIPS is a processor
architecture and nothing else; guesses on systems architecture are likely
to be wrong ...

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>