intel_adsp: ace: Fix heap in the linker

The end of the heap should be the same as _heap_sentry. The current end
marker just covers the range of memory that is explicitly put in
.heap_memand not account until the end of L2_SRAM_BASE +
L2_SRAM_SIZE memory.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>