mpc5xxx: Add stub implementation of cache functions

Some drivers (e.g. net/e1000) reference these functions. So, this
fixes the build of MVBC_P board.

I'm not familiar with the MPC5xxx platform, maybe a full
implementation shall be implemented instead of this stub in the
future.

Signed-off-by: Vasili Galka <vvv444@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>, Marek Vasut <marex@denx.de>
Acked-by: Wolfgang Denk <wd@denx.de>