juno: Use the enable_mmu_elX() functions from lib/aarch64/xlat_tables.c ...

Juno code can re-use the enable_mmu_elX() functions from
lib/aarch64/xlat_tables.c.

Because the Juno port now includes plat/common/aarch64/plat_common.c
the juno specific 'plat_common.c' needs to be renamed to avoid
name conflicts at build time.

Change-Id: I1e9cc6484643fbf516dfac0236573e0474ccc0a2