board: axs10x switch serial port and Ethernet to driver model

With this change Synopsys DesignWare SDP board is switched to driver
model for both serial port (serial_dw) and Ethernet (Designware GMAC).

This simplifies include/configs/axs101.h and allows for reuse of Linux's
Device Tree description.

For simplicity Linux's .dts files are not blindly copied but only very
few extracts of them are really used (those that are supported in U-Boot
at the moment).

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>