ARM: dts: am335x-pocketbeagle: Fix mmc0 Write Protect

ARM: dts: am335x-pocketbeagle: Fix mmc0 Write Protect

AM3358 pin mcasp0_aclkr (ZCZ ball B13) [0] is routed to P1.31 header [1]
Mode 4 of this pin is mmc0_sdwp (SD Write Protect).  A signal connected
to P1.31 may accidentally trigger mmc0 write protection.  To avoid this
situation, do not put mcasp0_aclkr in mode 4 (mmc0_sdwp) by default.

[0] http://www.ti.com/lit/ds/symlink/am3358.pdf
[1] https://github.com/beagleboard/pocketbeagle/wiki/System-Reference-Manual#531_Expansion_Headers

Fixes: 047905376a16 (ARM: dts: Add am335x-pocketbeagle)
Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>