topology: Expand attribute references inside $[] expressions

topology: Expand attribute references inside $[] expressions

Properly expand referred object attributes inside $[] expression. This
allows for example this simplified case:

Define {
       CHANNELS 2
       BIT_DEPTH 16
}

Object.Base.foo {
	channels	$CHANNELS
	sample_bits	$BIT_DEPTH
	frame_bits	"$[$channels * $sample_bits]"
}

Closes: https://github.com/alsa-project/alsa-utils/pull/250
Reported-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>
Signed-off-by: Jyri Sarha <jyri.sarha@linux.intel.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>