pia-am335x: rework UART deactivation settings

- unfortunately setting CONFIG_BOOTDELAY to 0 or removing the define
  doesn't disable the UART input evaluation. If there was a char in the
  UART buffer, the boot process will still stop.
- Change the behaviour to use CONFIG_AUTOBOOT_KEYED with a 0 delay, which
  seems to work fine.

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
pia-am335x: fix NAND configuration

- replace CONFIG_PIA_NAND with PIA_NAND
- add missing MTDIDS_DEFAULT define
- fix duplicate defines warnings

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
Merge branch 'pia-am335x-mmi-dev' into pia-am335x-dev

* pia-am335x-mmi-dev:
  pia-am335x: fix NAND configuration
  pia-am335x: rework UART deactivation settings

Conflicts:
	include/configs/am335x_pia.h
pia-am335x: update DDR3 memory configuration for MMI

- update settings according to PHY Leveling algorithms and PCB layout
  for MMI
- keep defaults for unspecified boards at the TI default settings

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>
pia-am335x: add missing pieces for NAND support

- add missing defines to allow booting via NAND
- remove defines already in generic TI family configs
- remove use board_nand_init() from board file

Signed-off-by: Björn Krombholz <b.krombholz@pironex.de>