ARM: at91/Kconfig: change at91sam9g45 entry

The AT91SAM9G45 entry covers the whole family so we also add the AT91SAM9M10
name and the "families" qualifier. Then, add a comment to explain which SoCs
are supported by this entry: AT91SAM9G45, AT91SAM9G46 but also
AT91SAM9M10 and AT91SAM9M11.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>