cmake: move ti_lm3s6965-specific code into the soc directory

As far as possible SoC-specific code should be contained to it's
directory and core systems should not be referencing specific
SoCs. This keeps the common code clean and makes it easier to maintain
out-of-tree SoCs.

To this end we move CONFIG_SOC_TI_LM3S6965_QEMU's soft-reset related
code out of arch/arm/core/cortex_m and into arch/arm/soc/ti_lm3s6965.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>