mmc: bcm2835-sdhost: use Host Software Queueing mechanism

See commit 511ce378 ("mmc: Add MMC host software queue support")

Introduced in 5.8, this feature lets the block layer issue up to 2
pending requests to the MMC layer which in certain cases can improve
throughput, but in the case of this driver can significantly reduce
context switching when performing random reads.

On bcm2837 with a performant class A1 card, context switches under FIO
random 4k reads go from ~8800 per second to ~5800, with a reduction in
hardIRQs per second from ~5800 to ~4000. There is no appreciable
difference in throughput.

For bcm2835, and for workloads other than random read, HSQ is a wash in
terms of throughput and CPU load.

So, use it by default.

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