arc: Move SYS_LITTLE_ENDIAN / SYS_BIG_ENDIAN selection to Kconfig

We can determine which of these we need given CPU_BIG_ENDIAN being
enabled or not, so move that logic to Kconfig from config.mk.

Cc: Alexey Brodkin <alexey.brodkin@synopsys.com>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Tom Rini <trini@konsulko.com>