gpio: make more generic

Instead of assuming we're running on the PinePhone (and therefore
hardcoding gpiochip labels and line numbers), make all of those
configurable. Legacy config files will still be parsed as long as they
lack the `chips` key.

Existing config files are also updated to match the new config format.

data/pine64,pinephone-1.0.toml

@@ -13,11 +13,12 @@ poweron_delay = 100000
 #recovery_timeout = 9
 
 [gpio]
-dtr = 358
-pwrkey = 35
-reset = 68
-apready = 231
-disable = 232
+chips = [ "1c20800.pinctrl", "1f02c00.pinctrl" ]
+dtr = { chip = 1, line = 6 }
+pwrkey = { chip = 0, line = 35 }
+reset = { chip = 0, line = 68 }
+apready = { chip = 0, line = 231 }
+disable = { chip = 0, line = 232 }
 
 [at]
 uart = "/dev/ttyS2"