Fix "ppc/85xx: Clean up use of LAWAR defines" breakage

Commit 002741ae modified include/asm-ppc/mmu.h such that the LAWAR_
defines were only enabled for the 83xx platform, but they are also
needed on MPC512x system. Enabling these for E300 systems seems thus
more appropriate.

Signed-off-by: Wolfgang Denk <wd@denx.de>