mtd: spi-nor: don't build Cadence QuadSPI on non-ARM

This controller driver is used only on ARM but is mostly written
portably so it can build on other arch'es. Unfortunately, at least x86
doesn't provibe readsl()/writesl() accessors. We could possibly fix this
issue in the future by using io{read,write}32_rep() instead, but let's
just drop the architectures we aren't using for now.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>