edac: arm64: Check the fatal bit and mark the error as Double-bit

On a double-bit error, the PMU counters may increment and
trigger the PPI edac pmu overflow interrupt before the SPI for the
double bit interrupt triggers.
Check the FATAL bit of the cache error syndrome registers and mark
the error as double bit if it is set.

Change-Id: I5f992dac877b3d8936f11483d29b4e82fc70b0bf
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>