am335x-fb: setup display PLL

The LCDC IP-core an be feed from several clock sources, one of those is
a dedicated DPLL for generating a dividable base-clock for this IP-core.

The TRM specifies the maximum input frequency for the LCCD with 200 MHz,
so we must not exceed this value with the PLL frequency (which can lock
much higher).

This patch tries every combination of multipliers and divisors of the
PLL and the IP-core itself for getting as near as possible the the
requested panel->pxl_clk.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Anatolij Gustschin <agust@denx.de>