PCI: keystone: Enable compile-testing on !ARM

Currently the Keystone driver can only be compile-tested on ARM, but
this restriction seems unnecessary. Get rid of it to increase test
coverage.

Build-tested with allyesconfig on x86, ppc, mips and riscv.

Link: https://lore.kernel.org/r/20200906195128.279342-1-alex.dewar90@gmail.com


Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>