nvmem: Use NVMEM_DEVID_AUTO

It is reasonable to declare multiple nvmem blocks. Unless a unique 'id'
is passed in for each block there may be name clashes.

Avoid this by using the magic token NVMEM_DEVID_AUTO.

Fixes: 5a3fa75a ("nvmem: Add driver to expose reserved memory as nvmem")

Signed-off-by: Phil Elwell <phil@raspberrypi.com>