The following patch is a first step to convert the 'struct pad_desc'
to a bitmapped cookie to facilitate adding platform specific pullup or
drive strength definitions to existing pad definitions without need to
rewrite the complete pad def.

The patch wraps 'struct pad_desc' in an opaque data type and
introduces macros to access the individual members.
This patch does not constitute any functional change!
Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>