add am335x_evm_z1_defconfig for PL Hummingbird Z1.x

Signed-off-by: Guillaume Tucker <guillaume.tucker@plasticlogic.com>
am335x_evm_defconfig: revert to original version without PL extras

Remove all PL extra settings to revert to the original BeagleBone
configuration.  This file was used as the default PL Hummingbird
Z1.x configuration for a while, which is now in
am335x_evm_z1_defconfig instead.

Signed-off-by: Guillaume Tucker <guillaume.tucker@plasticlogic.com>
am335x_evm_z6_z7_defconfig: do not use S1D13541 I2C master

As there appears to be some hardware-related issues when using
the S1D13541 I2C bus master, it is now disabled in the default
configuration file for Hummingbird Z6 and Z7 boards.

Signed-off-by: Guillaume Tucker <guillaume.tucker@plasticlogic.com>