nvmem: Add driver to expose reserved memory as nvmem

Firmware/co-processors might use reserved memory areas in order to pass
data stemming from an nvmem device otherwise non accessible to Linux.
For example an EEPROM memory only physically accessible to firmware, or
data only accessible early at boot time.

In order to expose this data to other drivers and user-space, the driver
models the reserved memory area as an nvmem device.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>

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Changes since v1:
 - Remove reserved memory phandle indirection by directly creating a
   platform device from the reserved memory DT node
 - Only map memory upon reading it to avoid corruption
 - Small cosmetic cleanups