Use ARM LDM instead of VFP for uncached reads on Marvell PJ4

Marvell PJ4 core used in CuBox very poorly handles VFP uncached
reads from the framebuffer. Using WMMX or ARM LDM reads is much
faster, with LDM instructions having a minor advantage. This
improves framebuffer read performance from ~50MB/s to ~100MB/s.

WMMX runtime detection and PJ4 core identification is also added
as part of this fix.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>