The ARM_INSTANTIATE_LOCK macro defines a lock meant to be further
manipulated using the arm_lock_init/get/release() macros. It has 2
variants, depending on the BL image it is compiled for. One version
defines the lock variable with internal linkage whereas the other one,
with external linkage.

Code that uses these macros is not compliant with MISRA rule 8.4 because
when using the external linkage version, there is no visible declaration
for the lock variable.

This patch defines the arm_lock variable with internal linkage in both
cases. This fits well the way these macros are used in the code today,
where the lock is not used outside of the translation unit it is
defined in.

Change-Id: I213a74a2a6088a4f1e9a61a319ca7579c2001320
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>