linker: Split nocache memory sections into loadable and non-loadable

Commit f9168ae4 made all non-cached memory
loadable by default.

However as nocache memory is typically used for reserving larger buffers to
be shared between peripherals, this comes at fairly large cost towards ROM
usage.

This commit creates two distinct sections for both loadable and
non-loadable nocache memory sections.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>