dts: bindings: clean up redundant required false attributes

DTS property attributes are (by default) not required.

Explicitly specifying `required: false` is redundant.
Perhaps a warning to that effect would be useful.

Signed-off-by: Chris Friedt <cfriedt@meta.com>

dts/bindings/riscv/riscv,cpus.yaml

@@ -6,7 +6,6 @@ include: cpu.yaml
 properties:
   mmu-type:
     description: Memory Management Unit (MMU)
-    required: false
     type: string
     enum:
       - riscv,sv32

dts/bindings/riscv/sifive-common.yaml

@@ -8,5 +8,4 @@ include: riscv,cpus.yaml
 properties:
     hardware-exec-breakpoint-count:
       type: int
-      required: false
       description: Number of hardware break points