net: macb: Kconfig: Rename Atmel to Cadence

The help text makes it look like NET_VENDOR_CADENCE enables support for
Atmel devices, when in reality it's a driver written by Atmel that
supports Cadence devices.  This may confuse users that have this device
on a non-Atmel SoC.

The fix is just s/Atmel/Cadence/, but I did go and re-wrap the Kconfig
help text as that change caused it to go over 80 characters.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>