cmd_led: Extend led command to support blinking and more leds

This patch extends the U-Boot "led" command to support automatic blinking
by setting a blink frequency in milliseconds. Additionally the number of
supported LEDs is increased to 6 (0...5).

This will be used by the PCA9551 LED driver.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>