MAINTAINERS: Update SiFive driver maintainers

MAINTAINERS: Update SiFive driver maintainers

Add myself as a maintainer for the various SiFive drivers, since I have
been performing cleanup activity on these drivers and reviewing patches
to them for a while now. Remove Palmer as a maintainer, as he is focused
on overall RISC-V architecture support.

Collapse some duplicate entries into the main SiFive drivers entry:
 - Conor is already maintainer of standalone cache drivers as a whole,
   and these files are also covered by the "sifive" file name regex.
 - Paul's git tree has not been updated since 2018, and all file names
   matching the "fu540" pattern also match the "sifive" pattern.
 - Green has not been active on the LKML for a couple of years.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Acked-by: Paul Walmsley <paul.walmsley@sifive.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20240215234941.1663791-1-samuel.holland@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>