regulator: add regulator_get_linear_step() stub helper

The regulator header has empty inline functions for most interfaces,
but not regulator_get_linear_step(), which has just grown a user
that does not depend on regulators otherwise:

drivers/clk/tegra/clk-tegra124-dfll-fcpu.c: In function 'get_alignment_from_regulator':
drivers/clk/tegra/clk-tegra124-dfll-fcpu.c:555:19: error: implicit declaration of function 'regulator_get_linear_step'; did you mean 'regulator_get_drvdata'? [-Werror=implicit-function-declaration]
  align->step_uv = regulator_get_linear_step(reg);
                   ^~~~~~~~~~~~~~~~~~~~~~~~~
                   regulator_get_drvdata
cc1: all warnings being treated as errors
scripts/Makefile.build:278: recipe for target 'drivers/clk/tegra/clk-tegra124-dfll-fcpu.o' failed

Add the missing stub along the others.

Fixes: b3cf8d06 ("clk: tegra: dfll: CVB calculation alignment with the regulator")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>