dt-bindings: riscv: Add Supm extension description

dt-bindings: riscv: Add Supm extension description

Add description for the Supm extension. Supm indicates support for pointer
masking in user mode. Supm is mandatory for RVA23S64.

Add dependency check that Supm requires either Smnpm or Ssnpm.

The Supm extension is ratified in commit d70011dde6c2 ("Update to ratified
state") of riscv-j-extension.

Signed-off-by: Guodong Xu <guodong@riscstar.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>