cmd: gpio: Add `gpio read` subcommand

As explained in commit 4af2a33ee5b9 ("cmd: gpio: Make `gpio input`
return pin value again") the `gpio input` is used in scripts to obtain
the value of a pin, despite the fact that CMD_RET_FAILURE is
indistinguishable from a valid pin value.
To be able to detect failures and properly use the value of a GPIO in
scripts we introduce the `gpio read` command that sets the variable
`name` to the value of the pin. Return code of the `gpio read` command
can be used to check for CMD_RET_SUCCESS or CMD_RET_FAILURE.
CONFIG_CMD_GPIO_READ is used to enable the `gpio read` command.

Signed-off-by: Diego Rondini <diego.rondini@kynetics.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index d3abe3a..f580797 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -996,6 +996,13 @@
 	help
 	  GPIO support.
 
+config CMD_GPIO_READ
+	bool "gpio read - save GPIO value to variable"
+	depends on CMD_GPIO
+	help
+	  Enables the 'gpio read' command that saves the value
+	  of a GPIO pin to a variable.
+
 config CMD_PWM
 	bool "pwm"
 	depends on DM_PWM