brcm63xx: fix compilation on 3.14

Fixes the following issue:

drivers/gpio/gpio-bcm63xx.c: In function 'bcm63xx_gpio_probe':
drivers/gpio/gpio-bcm63xx.c:80:4: error: implicit declaration of function 'devm_kasprintf' [-Werror=implicit-function-declaration]

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 44986