sfc: Allocate SRAM between buffer table and descriptor caches at init time

sfc: Allocate SRAM between buffer table and descriptor caches at init time

Each port has a block of 64-bit SRAM that is divided between buffer
table and descriptor cache regions at initialisation time.  Currently
we use a fixed allocation, but it needs to be changed to support
larger numbers of queues.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>