power: vexpress: cleanup: use builtin_platform_driver

Use the helper macro for builtin drivers taht don't do anything special
in driver init. This removes some boilerplate code.

Link: https://lore.kernel.org/r/20200527112608.3886105-3-anders.roxell@linaro.org


Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>