clk: x86: add "mclk" alias for Baytrail/Cherrytrail

Due to timing requirements, TI and Conexant manage the audio
reference clock from their ASoC codec drivers using the "mclk"
string. This patch adds another lookup for the "pmc_plt_clk_3"
clock to avoid Intel-specific tests in those codec drivers and
use code as-is.

To avoid a leak, clk_add_alias() is not used in this patch.
Instead the lookup is created manually as part of the .probe()
step and dropped in the .remove() step.

"pmc_plt_clk_3" is used exclusively for audio on all known
Baytrail/CherryTrail designs and is e.g. routed on the MCLK
(pin 26) of the MinnowBoardMAX Turbot LSE connector.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>