soc/intel_adsp: Remove magic entry point addresses

The platform layer (except cAVS 1.5, which computed it via alternate
means as an offset on other stuff) was specifying the entry point as
an explicit address needlessly.  In fact the linker scripts already
are written to place the entry point at the first address of linkable
RAM, which is already available as the RAM_BASE symbol.

Unify.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>