[ARM] Ensure deselected config variables are defined to 'n'

[ARM] Ensure deselected config variables are defined to 'n'
To keep the Config.in files relatively clean, we use the
following construct:

 if [ "$CONFIG_ARM" = "y" ]; then
    dep_tristate 'Foo' CONFIG_FOO $CONFIG_BAR
 fi

where CONFIG_BAR is some machine implementation or high-level
chip support configuration option.  If CONFIG_BAR is left
empty, then the tristate is offered to the user, which isn't
what we want.  Defining CONFIG_BAR to 'n' prevents the option
being offered.

This is a rule I generally try to implement within
arch/arm/config.in.

This cset makes CONFIG_SA1111 and CONFIG_ARM_THUMB behave that
way.