ARM: am335x_pia: disable PIA_KM defines and do switching via boards.cfg

- PIA_E2 and PIA_MMI are defined depending on board configuration
- use am335x_pia_config for E2 board
- use am335x_pia_mmi_config for MMI board

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