uboot-phytec

arc: select cache settings via menuconfig

This change allows to keep board description clean and minimalistic.
This is especially helpful if one board may house different CPUs with
different features.

It is applicable to both FPGA-based boards or those that have CPUs
mounted on interchnagable daughter-boards.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>