dma: imx-dma: Fix kernel crash due to missing clock conversion

With the new i.MX clock infrastructure we need to request the dma clocks
seperately: ahb and ipg clocks.

This fixes the following kernel crash and make audio to be functional again:

root@freescale /home$ aplay audio48k16S.wav
Playing WAVE 'audio48k16S.wav' : Signed 16 bit Little Endian, Rate 48000 Hz, Stereo
Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = c7b74000
[00000000] *pgd=a7bb5831, *pte=00000000, *ppte=00000000
Internal error: Oops: 17 [#1] PREEMPT ARM
Modules linked in:
CPU: 0    Not tainted  (3.5.0-rc5-next-20120702-00007-g3028b64 #1128)
PC is at snd_dmaengine_pcm_get_chan+0x8/0x10
LR is at snd_imx_pcm_hw_params+0x18/0xdc
pc : [<c02d3cf8>]    lr : [<c02e95ec>]    psr: a0000013
sp : c7b45e30  ip : ffffffff  fp : c7ae58e0
r10: 00000000  r9 : c7ae981c  r8 : c7b88800
r7 : c7ae5a60  r6 : c7ae5b20  r5 : c7ae9810  r4 : c7afa060
r3 : 00000000  r2 : 00000001  r1 : c7b88800  r0 : c7afa060
Flags: NzCv  IRQs ...