mtd: st_spi_fsm: only build for ARM
mtd: st_spi_fsm: only build for ARM

COMPILE_TEST allows us to build this driver on other arch'es. But not
all arch'es have the right I/O accessors -- particularly, x86 is missing
readsl() and writesl().

So just restrict this driver to ARCH_STI. It's still buildable for a
multiplatform ARM kernel, so it can get decent compile coverage.

Signed-off-by: Brian Norris <ac73eed4b8a8c9127255a2696c52b2e7503720b8@gmail.com>
Acked-by: Lee Jones <630e34333487a351a857f6b705e04d30b37c1629@linaro.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs