powerpc/83xx: Make serial ports work on MPC8315E-RDB w/ FSL U-Boots

FSL U-Boots use /soc8315@e0000000 node to search and fixup serial
nodes' clock-frequency properties. Though in upstream kernels we use
new naming convention -- for IMMR address space dts files specify
/immr@e0000000 nodes.

This makes FSL U-Boots fail to fixup the clock frequencies, and that
leads to serial ports misbehaviour. We can workaround the issue by
filling the clock frequency values manually.

p.s. For the same reason FSL U-Boots fail to fixup MAC addresses for
ethernet nodes, so users should either change the .dts file locally
or set MAC address via `ifconfig hw ether' command.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>