Workaround for Cortex A77 erratum 1925769

Workaround for Cortex A77 erratum 1925769

Cortex A77 erratum 1925769 is a Cat B erratum, present in older
revisions of the Cortex A77 processor core.  The workaround is to
set bit 8 in the ECTLR_EL1 register, there is a small performance cost
(<0.5%) for setting this bit.

SDEN can be found here:
https://documentation-service.arm.com/static/5f7c35d0d3be967f7be46d33

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: I9cf0e0b5dc1e3e32e24279d2632c759cc7bd7ce9