plat/arm: Fix ARM_INSTANTIATE_LOCK syntax anomaly

plat/arm: Fix ARM_INSTANTIATE_LOCK syntax anomaly

The current definition of ARM_INSTANTIATE_LOCK macro includes a
semicolon, which means it's omitted where it's used. This is anomalous
for a C statement in global scope.

Fix this by removing semicolon from the definition; and where it's a
NOP, declare a file-scoped variable explicitly tagged as unused to avoid
compiler warning.

No functional changes.

Change-Id: I2c1d92ece4777e272a025011e03b8003f3543335
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>