arm64: renesas: salvator-x: Sound SSI PIO support

This patch adds PIO sound support for Salvator-X board.
It can use 44.1kHz base sound only at this point, since 48kHz base
sound needs CS2000, but it is not yet upstreamed.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>