The VExpress power-off/reset driver is not needed on 64-bit platforms as
PSCI power-off and reset can be used instead. Stop selecting it so it
can be disabled and not always built-in.

CONFIG_VEXPRESS_CONFIG can also be dropped as it was a dependency for
CONFIG_POWER_RESET_VEXPRESS.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>