devres: provide devm_kmemdup_const()

devres: provide devm_kmemdup_const()

Provide a function similar to devm_strdup_const() but for copying blocks
of memory that are likely to be placed in .rodata.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>