The ICST binding now also supports 'reg' in addition to 'vco-offset' for
the VCO register address. Add support to the driver to get the VCO
address from 'reg'.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-clk@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210913192816.1225025-6-robh@kernel.orgReviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>