MIPS: don't include <linux/genhd.h> in <asm/mach-rc32434/rb.h>

MIPS: don't include <linux/genhd.h> in <asm/mach-rc32434/rb.h>

There is no need to include genhd.h from a random arch header, and not
doing so prevents the possibility for nasty include loops.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210727055646.118787-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>