target: armv7a: use proper type for struct armv7a_cache_common::outer_cache

target: armv7a: use proper type for struct armv7a_cache_common::outer_cache

The field 'outer_cache' is always initialized and used as a
pointer to 'struct armv7a_l2x_cache'.
There is no reason for using type 'void *' for it.

Change the type of 'outer_cache'.
Drop the useless cast while reading 'outer_cache'.

Change-Id: Iaea9d02e247da26e230f887c85fbf8e9d7be34d5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8858
Tested-by: jenkins