Add option to use symlink for touchDevice in QT_QPA_EGLFS_KMS_CONFIG

Sometimes in multi screen environment it is needed to set a symlink to the inputDevice since the mapping of screen and input event device node cannot be guaranteed. The parsing of the QT_QPA_EGLFS_KMS_CONFIG could take into account that the inputDevice can be a symlink and detect that and resolve the symlink and use that for the device node for inputDevice.