[PATCH] sata_sil24: rename PORT_PRB to PORT_LRAM and add PORT_LRAM_SLOT_SZ

PORT_PRB is a misnomer as the area also contains other stuff.  Rename
it to PORT_LRAM and add PORT_LRAM_SLOT_SZ.

Signed-off-by: Tejun Heo <htejun@gmail.com>