led: implement LED activity API

led: implement LED activity API

Implement LED activity API similar to BOOT LED API.

Usual activity might be a file transfer with TFTP, a flash write...

User of this API will call led_activity_on/off/blink() to signal these
kind of activity.

New Kconfig is implemented similar to BOOT LED, LED_ACTIVITY to
enable support for it.

It's introduced a new /options/u-boot property "activity-led" and
"activity-led-period" to define the activity LED label and the
default period when the activity LED is set to blink mode.

If "activity-led-period" is not defined, the value of 250 (ms) is
used by default.

If CONFIG_LED_BLINK or CONFIG_LED_SW_BLINK is not enabled,
led_boot_blink call will fallback to simple LED ON.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>