ARM: dts: am43xx: fix SLEWCTRL_FAST pinctrl binding

According to AM437x TRM, Document SPRUHL7B, Revised December 2014,
Section 7.2.1 Pad Control Registers, setting bit 19 of the pad control
registers actually sets the SLEWCTRL value to slow rather than fast as
the current macro indicates. Introduce a new macro, SLEWCTRL_SLOW, that
sets the bit, and modify SLEWCTRL_FAST to 0 but keep it for
completeness.

Current users of the macro (i2c, mdio, and uart) are left unmodified as
SLEWCTRL_FAST was the macro used and actual desired state. Tested on
am437x-gp-evm with no difference in software performance seen.

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