clk: mvebu: armada-38x: add support for 1866MHz variants

The Linksys WRT3200ACM CPU is clocked at 1866MHz. Add 1866MHz to the
list of supported CPU frequencies. Also update multiplier and divisor
for the l2clk and ddrclk.

Noticed by the following warning:
[    0.000000] Selected CPU frequency (16) unsupported

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>