riscv: use invalidate/flush_*cache_range functions in cache.c

The flush_cache() function in lib/cache.c ignores its arguments and
flushes the complete data and instruction caches. Use the
invalidate/flush_*cache_range() functions instead to only flush the
requested memory region.

This patch does not change the current behavior of U-Boot, since the
implementation of the invalidate/flush_*cache_range() functions flush
the complete data and instruction caches. It is in preparation for CPUs
with the necessary functionality for flushing a selectable memory range.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>