qt: QT_COORD_TYPE_DOUBLE should only be set on soft float

If QT_COORD_TYPE_DOUBLE is forced and hard float is used, Qt compilation
breaks because it can't find hard coded operations on double.

Moreover, QT_COORD_TYPE is float only if QT_NO_FPU is set.
(cf src/corelib/global/qglobal.h)

So, we can safely make the QT_COORD_TYPE_DOUBLE option depend on
BR2_SOFT_FLOAT.

Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>