[ACPI] remove "default m" from acpi/Kconfig

Andi Kleen suggested it was unconventional for us to "default m"
on ACPI modules -- even though they are expected to be deployed
as modules.  But as "default n" would likely result in some
users building nonsense kernels, we compromise to "default y".

Distros are expected to continue to use =m in their configs.

Signed-off-by: Len Brown <len.brown@intel.com>