ARM: davinci: only use NVMEM when available

The davinci platform contains code that calls into the nvmem
subsystem, but that might be a loadable module, causing a
link error:

arch/arm/mach-davinci/built-in.o: In function `davinci_get_mac_addr':
:(.text+0x1088): undefined reference to `nvmem_device_read'
arch/arm/mach-davinci/built-in.o: In function `read_factory_config':
:(.text+0x214c): undefined reference to `nvmem_device_read'

Also, when NVMEM is completely disabled, the functions fail with
nonobvious error messages.

This ensures we only call the API functions when the code is actually
reachable from the board file, and otherwise prints a unique log
message.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: bec3c11b ("misc: at24: replace memory_accessor with nvmem_device_read")
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>