ARM: dts: spear3xx: Fix gmac node

On SPEAr3xx, ethernet driver is not compatible with the SPEAr600
one.
Indeed, SPEAr3xx uses an earlier version of this IP (v3.40) and
needs some driver tuning compare to SPEAr600.

The v3.40 IP support was added to stmmac driver and this patch
fixes this issue and use the correct compatible string for
SPEAr3xx

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>