Asoc: sam9x5_wm8731: depend on ARCH_AT91 instead of SOC_AT91SAM9X5

The SOC_AT91SAM9X5 option is going to be removed, only depend on ARCH_AT91 like
for the other drivers.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>