powerpc/85xx: Rename p2040_rdb.c to p2041_rdb.c

There's only p2041rdb board for official release, but the p2041 silicon
on the board can be converted to p2040 silicon without XAUI and L2 cache
function, then the board becomes p2040rdb board. so we use the file name
p2041_rdb.c to handle P2040RDB board and P2041RDB board which is also
consistent with the board name under U-Boot.

During the rename we make few other minor changes to the device tree:
* Move USB phy setting into p2041si.dtsi as its SoC not board defined
* Convert PCI clock-frequency to decimal to be more readable

Signed-off-by: Mingkai Hu <Mingkai.hu@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>