nios2: convert cache flush to use dm cpu data

Convert cache flush to use dm cpu data.

The original cache flush functions are written in assembly
and use CONFIG_SYS_{I,D}CACHE_SIZE... macros. It is difficult
to convert to use cache configuration in dm cpu data which is
extracted from device tree.

The cacheflush.c of Linux nios2 arch uses cpuinfo structure,
which is very close to our dm cpu data. So we copy and modify
it to arch/nios2/lib/cache.c to replace the old cache.S.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>