ARM: shmobile: mackerel: Use pinconf API to configure pin pull-down

The USB0 and USB1 VBUS pins must be pulled down. Add corresponding
configuration entries in the pinctrl map table instead of manually
poking the pin control registers.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>