net: sh_eth: arm: renesas: README: Drop CFG_SH_ETHER_CACHE_*

net: sh_eth: arm: renesas: README: Drop CFG_SH_ETHER_CACHE_*

Drop CFG_SH_ETHER_CACHE_WRITEBACK and CFG_SH_ETHER_CACHE_INVALIDATE,
which are now always enabled in the sh_eth driver, because those cache
operations are always available. On architectures which do not implement
cache operations yet, cache operations have to be implemented first.

CFG_SH_ETHER_ALIGNE_SIZE now set as SH_ETHER_ALIGN_SIZE in sh_eth.h
based on architecture and no longer configured on board level.

Remove CFG_SH_ETHER_CACHE_WRITEBACK configuration option from README.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>