arch: common: Make nocache region loadable

The `nocache` is not loadable, thus data stored therein cannot be
initialized by the startup code. This might be needed in special
cases. E.g. One might have a buffer which one wants to DMA into,
and which is a member of a struct. Other members of the struct one
may want to have initialized by the startup code.
The buffer thus should be placed in the `nocache` region, but for
the other members of the buffer to be initialized by the startup
code, the `nocache` region needs to be loadable.

Fix it by making the `nocache` region loadable. Adding a KConfig
symbol to do this optionally was considered, but deemed unnecessary
during the PR.

Signed-off-by: Julian Achatzi <mail@achatzi.pro>