ns16550: gate all PCI code with CONFIG_X86

ns16550: gate all PCI code with CONFIG_X86

The NS16550 driver is assuming that NS16550 PCI card are usable if the
architecture supports PCI (i.e. CONFIG_HAS_PCI=y). However, the code is
very x86 focus and will fail to build on Arm (/!\ it is not all the
errors):

ns16550.c: In function ‘ns16550_init_irq’:
ns16550.c:726:21: error: implicit declaration of function ‘create_irq’;
did you mean ‘release_irq’? [-Werror=implicit-function-declaration]
          uart->irq = create_irq(0, false);
                      ^~~~~~~~~~
                      release_irq
ns16550.c:726:21: error: nested extern declaration of ‘create_irq’
[-Werror=nested-externs]
ns16550.c: In function ‘ns16550_init_postirq’:
ns16550.c:768:33: error: ‘mmio_ro_ranges’ undeclared (first use in this
function); did you mean ‘mmio_handler’?
               rangeset_add_range(mmio_ro_ranges, uart->io_base,
                                  ^~~~~~~~~~~~~~
                                  mmio_handler
ns16550.c:768:33: note: each undeclared identifier is reported only once
for each function it appears in
ns16550.c:780:20: error: variable ‘msi’ has initializer but incomplete
type
              struct msi_info msi = {
                     ^~~~~~~~

Enabling support for NS16550 PCI card on Arm would require more plumbing
in addition to fixing the compilation error.

Arm systems tend to have platform UART available such as NS16550, PL011.
So there are limited reasons to get NS16550 PCI support for now on Arm.

Guard all remaining PCI code that is not under x86 flag with CONFIG_X86.

No functional change intended.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>