arm: mvebu: Espressobin: Set the maximum slave SPI speed to 40MHz

While the SPI controller speed is defined by DTS, the maximum
slave speed (connected devices) is limited by the pre-defined
configuration value CONFIG_SF_DEFAULT_SPEED to 1MHz
This patch increases this maximum SPI slave device speed to 40MHz

Change-Id: I0d1239bd8a2061c66725c2c227c1e1f49c92c29e
Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Reviewed-on: http://vgitil04.il.marvell.com:8080/59516


Tested-by: iSoC Platform CI <ykjenk@marvell.com>
Reviewed-by: Igal Liberman <igall@marvell.com>
[pali: Set CONFIG_SF_DEFAULT_SPEED via defconfig]
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>