clk: twl6040: Rename the driver and use consistent names in the code

The driver is to provide the functional clock to OMAP4/5 McPDM. The clock
is named as pdmclk in the documentations so change the function names,
structure names and variables to align with this.
At the same time rename the driver from "twl6040-clk" to "twl6040-pdmclk".
This can be done w/o regression since the clock driver is not in use at
the moment, the MFD core driver is not even registering the device for it.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>