ARM: keystone2: Cleanup PLL init code

There are two types of PLL for all keystone platforms:
Main PLL, Secondary PLL. Instead of duplicating the same definition
for each secondary PLL, have a common function which does
initialization for both PLLs. And also add proper register
definitions.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>