mfd: vexpress-sysreg: Initialize 'site' variable

'site' variable should be initialized with 0 so that when 'site'
property doesn't exist in DTB it can be handled correctly.

'0' value means board site number is motherboard (see
Documentation/devicetree/bindings/arm/vexpress.txt for details).

Signed-off-by: Semen Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>