[MIPS] Cleanup memory managment initialization.

Historically plat_mem_setup did the entire platform initialization.  This
was rather impractical because it meant plat_mem_setup had to get away
without any kind of memory allocator.  To keep old code from breaking
plat_setup was just renamed to plat_setup and a second platform
initialization hook for anything else was introduced.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>