ppc4xx: Update lwmon5 board support

This patch includes the following changes for the lwmon5 board support:

- Enable cache in SDRAM
- Use common EHCI driver instead of the PPC4xx specific OHCI driver
  This can be done since only high-speed devices are connected.
- Remove cached TLB entry again after ECC setup
- Use correct define for cache enabling
  (CONFIG_4xx_DCACHE instead of CONFIG_SYS_ENABLE_SDRAM_CACHE)
- Enable FIT image support

Signed-off-by: Stefan Roese <sr@denx.de>