clk: meson: s4: pll: determine maximum register in regmap config

When the max_register value is not set, the regmap debugfs 'registers'
file does not display the entire range of the regmap.

Signed-off-by: Dmitry Rokosov <ddrokosov@salutedevices.com>
Link: https://lore.kernel.org/r/20240320155512.3544-5-ddrokosov@salutedevices.com


Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>