drivers: thermal: virtual-sensor: update the virtual sensor name

Update the virtual sensor names from silver-virt-max-usr and
gold-virt-max-usr to silv-virt-max-step and gold-virt-max-step
respectively.

The name change reflects that the virtual sensors will be using the
step_wise governor instead of userspace governor.

Change-Id: I57de42315f2638c3f4b78ca79d9597b568797d77
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>