linux-imx

kconfig: Clarify choice dependency propagation

It's easy to miss that choices are special-cased to pass on their mode
as the parent dependency.

No functional changes. Only comments added.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>