ASoC: sgtl5000: Allow SCLK pad drive strength to be changed

This patch introduces "sclk-strength" property to allow SCLK pad drive
strength to be changed via device tree.

When running playback test on LS1028ARDB, Tx Frame sync error interrupt
will occur sometimes. Some noises also exist. After changing SCLK pad
drive strength to the maximum value, the issues are gone.

Signed-off-by: Alison Wang <alison.wang@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>