ARM: dts: am43xx: Fix inverted DS0_PULL_UP_DOWN_EN macro

Due to a mistake in documentation the DS0_PULL_UP_DOWN_EN macro was
mistakenly defined as an active high bit, however setting the bit
actually disables the internal pull resistor on the pin, so correct this
macro and introduce a new DS0_PULL_UP_DOWN_DIS macro with the proper bit
value set now that the documentation has been updated.

Change based on AM437x Techninal Reference Manual SPRUHL7G Revised June
2017 Section 7.2.1.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>