ARM: dts: am33xx: fix SLEWCTRL_FAST pinctrl binding

Copy missing Kernel patch to make SLEWCTRL_SLOW available in device tree
and to correct the value of SLEWCTRL_FAST.
After this patch, the Kernel and U-boot device tree settings for
SLEWCTRL have the same effect.

Original Kernel patch message:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=424e0f039bfa8a51fb5c5178b6ece8baa4996469

"
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>
"

Signed-off-by: Christian Amann <Christian.Amann@de.bosch.com>

include/dt-bindings/pinctrl/am33xx.h

@@ -13,7 +13,8 @@
 
 #define PULL_DISABLE		(1 << 3)
 #define INPUT_EN		(1 << 5)
-#define SLEWCTRL_FAST		(1 << 6)
+#define SLEWCTRL_SLOW		(1 << 6)
+#define SLEWCTRL_FAST		0
 
 /* update macro depending on INPUT_EN and PULL_ENA */
 #undef PIN_OUTPUT