bus: brcmstb_gisb: save and restore GISB timeout

When the system enters S3, we will lose the GISB timeout value we have
configured, make sure that we do save this timeout value, and restore
this timeout value prior to re-enabling interrupts such that the GISB
timeout interrupt will fire with the expected timeout.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>