net: ftmac100: change driver name from nds32_mac to ftmac100

So it will be named similarly to the related ftgmac100 driver.
The old name 'nds32_mac' is not referred to anywhere in U-Boot.

Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>