ARM: cns3xxx: shut up frame size warning

This shuts up a warning in the 3.18-stable series that has been fixed
in newer kernels with commit 498a92d4 ("ARM: cns3xxx: pci: avoid
potential stack overflow"):

arch/arm/mach-cns3xxx/pcie.c: In function 'cns3xxx_pcie_hw_init':
arch/arm/mach-cns3xxx/pcie.c:313:1: error: the frame size of 1080 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

The fix that went into v4.4 is known to be buggy and was later
fixed again with commit 88e9da9a ("CNS3xxx: Fix PCI
cns3xxx_write_config()"). While we could backport both to 3.18,
they are fairly invasive and the warning is definitely harmless
here as the call chain is known to not overflow the stack of the
init task.

This simply adds a Makefile flag to extend the limit for this one
file.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>