test(tc): test for AP/RSS NV counter interface

test(tc): test for AP/RSS NV counter interface

Change in PLATFORM_TEST build flag from boolean -> string, with the
current string options being tfm-testsuite and rss-nv-counters.

To get the old behavior, i.e. where we used to use PLATFORM_TEST=1,
we now need to pass PLATFORM_TEST=tfm-testsuite.

Adding new test of the AP/RSS interface for non-volatile counters.
The test reads, increments, and reads again each 3 types of NV
counters for: CCA, secure, and non-secure firmware. Enabled by
PLATFORM_TEST=rss-nv-counters.

Change-Id: I2044cc9b2f37984697e0754c9c824eab51a11e7f
Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
Signed-off-by: Raef Coles <raef.coles@arm.com>