blk: Invalidate block cache when switching hwpart

Some storage devices have multiple hw partitions and both address from
zero, for example eMMC.
However currently block cache invalidation only applies to block
write/erase.
This can cause a problem that data of current hw partition is cached
before switching to another hw partition. And the following read
operation of the latter hw partition will get wrong data when reading
from the addresses that have been cached previously.

To solve this problem, invalidate block cache after a successful
select_hwpart operation.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>