qt5jsbackend: only available on ARM, x86, x86-64 and mipsel

The v8 Javascript virtual machine that is the core of qt5jsbackend is
only available on certain architectures. At the moment, only ARM, x86,
x86-64 and mipsel are supported.

Fixes:

  http://autobuild.buildroot.org/results/a2e/a2e62e2deaa28ec9d4d957523a00ee0b7a634950/build-end.log



[Peter: also add dependency for qt5graphicaleffects]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>