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Some generic compatibility functions emit deprecated declaration warnings
even when platforms do not use the deprecated functions directly. This
can be confusing. Suppress these warnings by using:
`#pragma GCC diagnostic ignored "-Wdeprecated-declarations"`

Also emit a runtime warning if the weak plat/common implemntation of
plat_get_syscnt_freq2() is used, as this implies the platform has not
migrated from plat_get_syscnt_freq(). The deprecated  declaration warnings
only help detect when platforms are calling deprecated functions, not when
they are defining deprecated functions.

Fixes ARM-software/tf-issues#550

Change-Id: Id14a92279c2634c1e76db8ef210da8affdbb2a5d
Signed-off-by: Dan Handley <dan.handley@arm.com>