Add bits of SD support that depend on new APIs:

 - detect the readonly slider on the cards;
 - report when the hardware supports 4-bit wide reads;
 - enable 4-bit reads when the upper layer put the card into that mode
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>