[MTD] Fix dependencies for MIPS MTD drivers

 o A dependency on the processor architecture does not make sense;
   delete it.
 o The Alchemy and MTX drivers requires MTD_PARTITIONS and MTD_CFI to work,
   make those dependencies.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>