Fix MISRA Rule 5.7 Part 1

Fix MISRA Rule 5.7 Part 1

Rule 5.7: A tag name shall be a unique identifier

There were 2 amu_ctx struct type definitions:
    - In lib/extensions/amu/aarch64/amu.c
    - In lib/cpus/aarch64/cpuamu.c

Renamed the latter to cpuamu_ctx to avoid this name clash

To avoid violation of Rule 8.3 also change name of function
amu_ctxs to unique name (cpuamu_ctxs) since it now returns a
different type (cpuamu_ctx) than the other amu_ctxs function

Fixed for:
    make LOG_LEVEL=50 PLAT=fvp

Change-Id: Ieeb7e390ec2900fd8b775bef312eda93804a43ed
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>