PCI: imx6: Simplify Kconfig depends on

The imx6 driver can be used on imx6sx without enabling support for
imx6q or imx7d but the "depends on" condition doesn't allow that.

Instead of making the condition even longer just make it depend on
"ARCH_MXC || COMPILE_TEST" instead.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>