dts/bindings: Fix build issues with bindings

the microchip,mcp2515 and microchip,enc28j60 bindings would hit the
following build error:

device tree error: dts/bindings/can/microchip,mcp2515.yaml (in 'reg'):
'category' from !included file overwritten
('required' replaced with 'optional')

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>