arch/xtensa: adsp: Rename module_init section

.module_init sections is used to keep all components constructor
functions.

Zephyr uses -ffunction-sections option which will create a section for
each function. Unfortunately, this creates a section named .module_init
for the function module_init() used to initialize the processing module
generic layer.

Thus, places module_init() in the constructor area named .module_init
which is wrong.

To avoid this we rename .module_init section for constructors to
.initcall.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>