riscv: move sifive_l2_cache.c to drivers/soc

riscv: move sifive_l2_cache.c to drivers/soc

The sifive_l2_cache.c is in no way related to RISC-V architecture
memory management.  It is a little stub driver working around the fact
that the EDAC maintainers prefer their drivers to be structured in a
certain way that doesn't fit the SiFive SOCs.

Move the file to drivers/soc and add a Kconfig option for it, as well
as the whole drivers/soc boilerplate for CONFIG_SOC_SIFIVE.

Fixes: a967a289f169 ("RISC-V: sifive_l2_cache: Add L2 cache controller driver for SiFive SoCs")
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Borislav Petkov <[email protected]>
[[email protected]: keep the MAINTAINERS change specific to the L2$ controller code]
Signed-off-by: Paul Walmsley <[email protected]>