riscv + sbi_serial + mtd-ram

[Marcelo Politzer]

Hi,

I am currently working on a riscv platform that:
1) uses the sbi putchar / getchar as serial.
2) maps its `flash` directly to ram as `mtd-ram`[1] (snippet from device
tree provided by the ROM, forwarded by OpenSBI).

I successfully implemented 1) based on other serial drivers. But I would
like advice on point 2).
The IO part should be straightforward, just forwarding reads and writes to
the ram is enough.
I am having a hard time figuring out the plumbing (where should I implement
these functions).

Any thoughts would be appreciated.

Best,
Marcelo

[1]
```
flash at 8000000000000000 {
#address-cells = <0x02>;
#size-cells = <0x02>;
compatible = "mtd-ram";
bank-width = <0x04>;
reg = <0x80000000 0x00 0x00 0x3c00000>;
linux,mtd-name = "flash.0";
};
```