configs: Add CONFIG_BCACHE=m

Allow block devices to be used as caches for other devices. The primary
use is to allow small, low latency media to act as caches for spinning
rust drives.

See: https://github.com/raspberrypi/linux/issues/6303
     https://github.com/raspberrypi/linux/issues/455



Signed-off-by: Phil Elwell <phil@raspberrypi.com>