axs101: Enable data cache

There's no reason to keep data cache disabled in axs101
board any longer, enabling it.

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

configs/axs101_defconfig

 CONFIG_ARC=y
-CONFIG_SYS_DCACHE_OFF=y
 CONFIG_TARGET_AXS101=y
 CONFIG_SYS_TEXT_BASE=0x81000000
 CONFIG_SYS_CLK_FREQ=750000000