firmware: dmi: Add info message for number of populated and total memory slots

As part of adding support for calling i2c_register_spd() on muxed SMBUS
segments the same message has been removed from i2c_register_spd().
However users may find it useful, therefore reintroduce it as part of
the DMI scan code.

[JD: Static variable dmi_memdev_populated_nr is only used in __init
 functions, so it can be marked __initdata.]

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jean Delvare <jdelvare@suse.de>