ARM: keystone2: Cleanup init_pll definition

This is just a cosmetic change that makes
the calling of pll init code looks much cleaner.

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