MIPS: Enable ISA_DMA_API config to fix build failure

Add ISA_DMA_API config item and select it when GENERIC_ISA_DMA enabled.
This fixes build failure on allmodconfig like following:

  CC      sound/isa/es18xx.o
sound/isa/es18xx.c: In function 'snd_es18xx_playback1_prepare':
sound/isa/es18xx.c:501:9: error: implicit declaration of function 'snd_dma_program'
sound/isa/es18xx.c: In function 'snd_es18xx_playback_pointer':
sound/isa/es18xx.c:818:3: error: implicit declaration of function 'snd_dma_pointer'
make[3]: *** [sound/isa/es18xx.o] Error 1
make[2]: *** [sound/isa/es18xx.o] Error 2
make[1]: *** [sub-make] Error 2
make: *** [all] Error 2

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/1717/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -880,11 +880,15 @@ config NO_IOPORT
 config GENERIC_ISA_DMA
 	bool
 	select ZONE_DMA if GENERIC_ISA_DMA_SUPPORT_BROKEN=n
+	select ISA_DMA_API
 
 config GENERIC_ISA_DMA_SUPPORT_BROKEN
 	bool
 	select GENERIC_ISA_DMA
 
+config ISA_DMA_API
+	bool
+
 config GENERIC_GPIO
 	bool