machine: make machine.I2C0 and similar objects pointers

This makes it possible to assign I2C objects (machine.I2C0,
machine.I2C1, etc.) without needing to take a pointer.

This is important especially in the future when I2C may be driven using
DMA and the machine.I2C type needs to store some state.

src/machine/board_hifive1b_baremetal.go

@@ -10,10 +10,3 @@ var (
 		Bus: sifive.QSPI1,
 	}
 )
-
-// I2C on the HiFive1 rev B.
-var (
-	I2C0 = I2C{
-		Bus: sifive.I2C0,
-	}
-)