clk: vexpress-osc: Use the devres clock API variants

In preparation to enable the vexpress-osc clock driver as a module,
convert the driver to use the managed devres clock API variants. With
this, a driver .remove() hook is not needed.

Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-clk@vger.kernel.org
Reviewed-by: Stephen Boyd <sboyd@kernel.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>