Add a variant of shmem_read() that takes a iosys_map pointer rather
than a plain pointer as argument. It's mostly a copy __shmem_rw() but
adapting the api and removing the write support since there's currently
only need to use iosys_map as destination.

Reworking __shmem_rw() to share the implementation was tempting, but
finding a good balance between reuse and clarity pushed towards a little
code duplication. Since the function is small, just add the similar
function with a copy/paste/adapt approach.

v2: Add an offset as argument and instead of using a map iterator, use the
offset to keep track of where we are writing data to.

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220216174147.3073235-4-lucas.demarchi@intel.com