[ARM] Kirkwood: enhance TCLK detection

According to the Marvell LSP, the Sample at Reset regiter bit 21 can be
used to detect TCLK on 6281 and 6282 devices.

This patch has only been tested on LaCie boards.

Signed-off-by: Simon Guinot <sguinot@lacie.com>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>