MIPS: Purple: Remove Purple support

The Purple SoC and eval board are not actively maintained since years.
This patch removes the support completely as aggreed with Wolfgang Denk.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>