The assembler helper function `print_revision_warning` is used when a
CPU specific operation is enabled in the debug build (e.g. an errata
workaround) but doesn't apply to the executing CPU's revision/part number.
However, in some cases the system integrator may want a single binary to
support multiple platforms with different IP versions, only some of which
contain a specific erratum.  In this case, the warning can be emitted very
frequently when CPUs are being powered on/off.

This patch modifies this warning print behaviour so that it is emitted only
when LOG_LEVEL >= LOG_LEVEL_VERBOSE. The `debug.h` header file now contains
guard macros so that it can be included in assembly code.

Change-Id: Ic6e7a07f128dcdb8498a5bfdae920a8feeea1345