clk: versaclock3: Remove redundant _is_writeable()

The versaclock3 driver provides an _is_writeable() function which returns
true for all registers. This is the default assumption for regmaps so we
can remove the function for a very minor improvement in performance and
code size.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230929-clk-maple-versaclk-v1-1-24dd5b3d8689@kernel.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>