MAINTAINERS: add orphaned RISC-V boards back to the RISC-V area

This commit restores the pre-HWMv2 status of RISC-V boards being covered by
the RISC-V area of maintenance.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>

MAINTAINERS.yml

@@ -2965,6 +2965,8 @@ RISCV arch:
  - ycsin
  files:
  - arch/riscv/
+ - boards/enjoydigital/litex_vexriscv/
+ - boards/lowrisc/opentitan_earlgrey/
  - boards/qemu/riscv*/
  - boards/sifive/
  - boards/sparkfun/red_v_things_plus/