ARM: kirkwood: stop printk TCLK value at boot for DT boards

Commit 2326f043 (ARM: kirkwood: convert to DT irqchip and clocksource)
dropped the call to kirkwood_timer_init() when booting with DT.
This results in kirkwood_tclk not being set and the boot message is now:

Kirkwood: MV88F6281-A0, TCLK=0

This patch modifies the message for printing only the kirkwood id,
as it allows to detect if we're running A0/A1/... variants with a simple
dmesg.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>