drivers: mmc: preallocate a block for SD extension register accesses

The Performance Extension register is regularly accessed in a hot path
to do write cache flushes. Don't invoke kmalloc/kfree for every access,
preallocate a 512B buffer for this purpose.

Also remove an unused alloc in sd_enable_cache().

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>