libftdi: remove BR2_ARCH_HAS_ATOMICS dependency

In commit 669ce8c7 ("libftdi: add
dependency on atomic operations"), a dependency on
BR2_ARCH_HAS_ATOMICS was added to the libftdi package to fix a build
failure occuring on the ARC architecture due to the missing
__sync_fetch_and_add_4 function:

../ftdipp/.libs/libftdipp.so: undefined reference to `__sync_fetch_and_add_4'

However, today, even on the SPARC architecture that does not implement
any of the __sync built-ins, libftdi and its C++ binding libftdipp
build fine. ARC was also tested and builds fine.

Therefore, we remove the BR2_ARCH_HAS_ATOMICS dependency from libftdi,
and also from flashrom, in which it was only present due to the
selection of libftdi. Note that anyway flashrom is available only for
i386 and x86_64, both of which implement all the __sync built-ins.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>