da8xx: add support for multiple PLL controllers

Modify clk_get() function in cpu file to work for
multiple PLL controllers.

Signed-off-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Signed-off-by: Nagabhushana Netagunte <nagabhushana.netagunte@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>