Read-modify-write (RMW) module

[zarubaf]

In order to be able to protect an SRAM using ECC, only full-word accesses can be made as the parity word is assembled over the entire word. It is still beneficial to allow for byte-wise accesses though.

The idea is to create a module that contains the following interface on the in-side (minus atop):

snitch/hw/system/occamy/src/occamy_top.sv

Lines 1637 to 1645 in d598922

	.mem_req_o(spm_req),
	.mem_gnt_i(spm_req), // always granted - it's an SPM.
	.mem_addr_o(spm_addr),
	.mem_wdata_o(spm_wdata),
	.mem_strb_o(spm_strb),
	.mem_atop_o(),
	.mem_we_o(spm_we),
	.mem_rvalid_i(spm_rvalid),
	.mem_rdata_i(spm_rdata)

If the module detects a write access without the full write mask set a state machine should do a read access, followed by a partial merge (dependent on the write mask) and a consecutive write. The output interface of the module looks similar to the input interface minus the write-mask (which is implicitly set to '1).