pinctrl: aspeed: Fix documentation

Fixes these warnings:

  pinctrl-aspeed.c:112: warning: Function parameter or member 'map' not
  described in 'aspeed_sig_desc_eval'
  pinctrl-aspeed.c:112: warning: Excess function parameter 'regmap'
  description in 'aspeed_sig_desc_eval'

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>