pcm052: allow specifying onboard DDR size in configs

PCM052 SoMs may be equipped with various sizes of DDR.
Keep default of 256MB; new PCM052-based targets will
specify their actual DDR size.

Linux command line is auto-adjusted to DDR size.

Signed-off-by: Albert ARIBAUD (3ADEV) <albert.aribaud@3adev.fr>