soc: ti: k3: am6x: do not override KERNEL_ENTRY

The SOC defconfig overrides CONFIG_KERNEL_ENTRY from the default of
__start to _vector_table. This is undesirable for cores such as M4 where
the _vector_table symbol has just raw addresses and no instructions. The
change was done to make sure Zephyr images can be loaded via remoteproc in
which case the entrypoint needs to be 64 byte aligned. To fix this, use
_vector_table as the ELF entrypoint only for R5 cores.

Signed-off-by: Amneesh Singh <a-singh7@ti.com>