ARM/ixp4xx: Make NEED_MACH_IO_H optional

In order to create a proper PCI driver for the IXP4xx
we need to make the old PCI driver and its reliance
on <mach/io.h> optional.

Create a new Kconfig symbol for the legacy PCI driver
IXP4XX_PCI_LEGACY and only activate NEED_MACH_IO_H
for this driver.

A few files need to be adjusted to explicitly include
the <mach/hardware.h> and <mach/cpu.h> headers that
they previously obtained implicitly using <linux/io.h>
that would include <mach/io.h> and in turn include
these two headers.

This breaks our reliance on the old PCI and indirect
PCI support so we can reimplement a proper purely
DT-based driver in the PCI subsystem.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khalasa@piap.pl>
Cc: Zoltan HERPAI <wigyori@uid0.hu>
Cc: Raylynn Knight <rayknight@me.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>