regulator: vexpress: rename vexpress regulator implementation

The vexpress regulator implementation is currently just called vexpress.
This is a problem because it clashes with another module with the same
name in hardware monitors.

This patch renames the vexpress regulator implementation to
vexpress-regulator so that there will be no clash in the module namespace.

Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Reported-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>