conf/tune: add tune-ppce300c3

conf/tune: add tune-ppce300c3

It has been pointed out several times that the yocto mpc8315e-rdb
reference was using the wrong tuning (603e), since it is actually
a e300c3 board.

This commit creates a e300c3 tune file based on the e300c2 variant
already in oe-core.

This commit also inhibits altivec in flac when this new tuning is
enabled and used by the mpc8315e-rdb

[YOCTO #1192]

(From OE-Core rev: 8663c7ba0530eb36728fe524ed0137e064cc1c5a)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>