ARM: dts: am57xx-beagle-x15: Remove pinmux configurations for erratum i869

Pinmuxing for DRA7x/AM57x family of processors need to be done in IO
isolation as part of initial bootloader executed from SRAM. This is
done as part of iodelay configuration sequence and is required due
to the limitations introduced by erratum ID: i869[1] (IO Glitches
can occur when changing IO settings) and elaborated in the Technical
Reference Manual[2] 18.4.6.1.7 Isolation Requirements.

Only peripheral that is permitted for dynamic pin mux configuration
is MMC and DCAN. MMC is permitted to change to accommodate the
requirements for varied speeds (which require IO-delay support in
kernel as well). DCAN is a result of i893[1] (DCAN initialization
sequence). However, since we don't use DCAN on X15, with the exception
of MMC, all other pin mux configurations are removed from the dts.

[1] http://www.ti.com/lit/pdf/sprz429
[2] http://www.ti.com/lit/pdf/sprui30



Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>