[mem_bkdr_if] Rework write8 function to allow more memory widths
[mem_bkdr_if] Rework write8 function to allow more memory widths

This patch changes write8 to dispatch on the integrity type and then
the memory width and then simplifies the "no integrity" and "parity"
paths significantly, generalising them to allow arbitrary memory
widths.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan