ARM: dts: imx27: Use the correct usb clock gate

USB Host1, Host2 and OTG are gated via 'usb_ipg_gate' clock, so fix it in order
to avoid the following kernel oops:

usbcore: registered new interface driver usb-storage
10024000.usb supply vbus not found, using dummy regulator
Unhandled fault: external abort on non-linefetch (0x808) at 0xf4424184
Internal error: : 808 [#1] PREEMPT ARM
Modules linked in:
CPU: 0 PID: 1 Comm: swapper Not tainted 3.15.0-rc1-26325-g971f9fd-dirty #64
task: c7829aa0 ti: c7836000 task.ti: c7836000
PC is at ci_hdrc_probe+0x3a4/0x634
LR is at ci_hdrc_probe+0x100/0x634
pc : [<c036cc78>]    lr : [<c036c9d4>]    psr: 60000013
sp : c7837d48  ip : 00000001  fp : 00000000
r10: 00000000  r9 : 00000000  r8 : c791b6c0
r7 : c7945000  r6 : f4424000  r5 : c7945010  r4 : c794e010
r3 : f4424184  r2 : 00000000  r1 : 8c000004  r0 : 0c000004
Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
Control: 0005317f  Table: a0004000  DAC: 00000017
Process swapper (pid: 1, stack limit = 0xc78361c0)
Stack: (0xc7837d48 to 0xc7838000)

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>