core: add non-volatile monotonic counter interface

Adds a stubbed non-volatile monotonic counter interface with a REE FS
counter. Platforms or drivers overrides the weak functions
nv_counter_get_ree

core: add non-volatile monotonic counter interface

Adds a stubbed non-volatile monotonic counter interface with a REE FS
counter. Platforms or drivers overrides the weak functions
nv_counter_get_ree_fs() and nv_counter_incr_ree_fs_to() to provide a
non-stubbed implementation of the counter.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>

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