ARM: dts: am33xx: fix SLEWCTRL_FAST pinctrl binding

According to AM335x TRM, Document spruh73l, Revised February 2015,
Section 9.2.2 Pad Control Registers, setting bit 6 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 and mdio) are left unmodified as
SLEWCTRL_FAST was the macro used and actual desired state. Tested on
am335x-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>