soc: imx8m: adsp: Add NonCacheable linker section

There are some drivers like NXP SDMA that need a NonCacheable
data region to put data like channel control or buffer descriptors.

So far, we haven't added such a section because the linker created
one default orphan section at the end of the data section.

But this generates a warning in the build system:
xtensa-nxp_imx8m_adsp_zephyr-elf/bin/ld.bfd: warning: orphan section
NonCacheable' from modules/hal_nxp/libmodules__hal_nxp.a(fsl_sdma.c.obj)'
being placed in section `NonCacheable'

So fix this by explicitly define a NonCacheable area at the end of data
section.

This works because imx8mp cache attributes are
_memmap_cacheattr_imx8_wt_allvalid = 0x22212222
and the area where the NonCacheable section is allocated is
write-through.

So all the configuration for the SDMA core is not-cached at write.

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