dcache: Rework RAM wrapper to synthetize better on Xilinx

dcache: Rework RAM wrapper to synthetize better on Xilinx

The global wr_en signal is causing Vivado to generate two TDP (True Dual Port)
block RAMs instead of one SDP (Simple Dual Port) for each cache way. Remove
it and instead apply a AND to the individual byte write enables.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>