src/mainboard/pcengines/apu2/OemCustomize.c: Enable CPB feature

src/mainboard/pcengines/apu2/OemCustomize.c: Enable CPB feature

Enable Core Performance Boost feature in automatic mode.
Also enable C6 state which is a dependency for proper CPB operation.

CPB allows to raise single core frequency from 1000MHz to 1400MHz
during high load if other cores idle. The processor has additional
boosted P-states when CPB is enabled, but these are hidden from OS.

TEST: Higher single-core CPU performance is indicated by increased
memory bandwidth as reported by memtest86+.

Signed-off-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Change-Id: I5e080bfaee06fd13cedf5151d4a598ec212213f2
Reviewed-on: https://review.coreboot.org/c/31229
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>