arm64: dts: r8a7795: salvator-x: Set drive-strength for ravb pins

The EthernetAVB should not depend on the bootloader to setup correct
drive-strength values. Values for drive-strength where found by
examining the registers after the bootloader has configured the
registers and successfully used the EthernetAVB.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>