qt5webkit: disable for MIPS soft-float

It assumes the MIPS target has an FPU and uses FPU assembler
instructions which cause the compilation to fail when building it for
soft-float.

Fixes:
  http://autobuild.buildroot.net/results/f40/f407ca9245e2445619420a9dfd595856729a2b2b/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
[Thomas:
 - propagate dependency to Config.in comment
 - add comment above the "depends on" to explain why.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>