riscv: Weakly define invalidate_icache_range()

Some RISC-V CPUs, such as the T-HEAD XuanTie series, have a
vendor-specific way to invalidate a portion of the instruction cache.
Allow them to override invalidate_icache_range().

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>