rtc: test: make array pdev static

The array pdev is local to the source and does not need to be in
global scope, so make it static.

Cleans up sparse warning:
symbol 'pdev' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>