ARM: 7003/1: vexpress: Add clock definition for the SP805.

It seems that an entry for the SP805 watchdog in the table of clocks was
missing.  This results in the sp805_wdt driver rejecting the device with
the following errors:

  sp805-wdt mb:wdt: Clock not found
  sp805-wdt mb:wdt: Probe Failed!!!
  sp805-wdt: probe of mb:wdt failed with error -2

While not obviously stated in the hardware docs, the onboard SP810's
"REFCLK" is connected to a 32.768KHz crystal, and this drives the
watchdog.  Add a struct clk and corresponding lookup entry for it.

Signed-off-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>