net: thunderx: add 64-bit dependency

The thunderx ethernet driver fails to build on architectures
that do not have an atomic readq() and writeq() function for
64-bit PCI bus access:

drivers/net/ethernet/cavium/thunder/thunder_bgx.c: In function 'bgx_reg_read':
include/asm-generic/io.h:195:23: error: implicit declaration of function 'readq' [-Werror=implicit-function-declaration]

It seems impossible to get this driver to work on most 32-bit
hardware, so it's better to add an explicit dependency, in
order to let us keep building 'allmodconfig' kernels on
all architectures.

As the driver is meant for the internal hardware on an arm64 SoC, this
is not a problem for usability. Allowing the build on all 64-bit
architectures rather than just CONFIG_ARM64 on the other hand means that
we get the benefit of build testing on x86.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>