ARM: dts: stm32mp1: add pull-up on serial rx of console connected to STLINK

Avoid U-Boot auto-boot interruption for line break detection
on console when the RX line connected to STLINK is floating
(-IO error in getc cause by framing error and testc return 1)
Same workaround is applied on all the STMicroelectonics board.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>

arch/arm/dts/stm32mp157a-dk1-u-boot.dtsi

@@ -189,6 +189,8 @@
 	};
 	pins2 {
 		u-boot,dm-pre-reloc;
+		/* pull-up on rx to avoid floating level */
+		bias-pull-up;
 	};
 };
 

arch/arm/dts/stm32mp157c-ed1-u-boot.dtsi

@@ -206,5 +206,7 @@
 	};
 	pins2 {
 		u-boot,dm-pre-reloc;
+		/* pull-up on rx to avoid floating level */
+		bias-pull-up;
 	};
 };