OMAPDSS: fix registration of DPI and SDI devices

The omapdss arch initialization code registers all the output devices as
omap_devices. However, DPI and SDI are not proper omap_devices, as they
do not have any corresponding HWMOD. This leads to crashes or problems
when the platform code tries to use omap_device functions for DPI and
SDI devices.

One such crash was reported by John Stultz <johnstul@us.ibm.com>:

[   18.756835] Unable to handle kernel NULL pointer dereference at
virtual addr8
[   18.765319] pgd = ea6b8000
[   18.768188] [00000018] *pgd=aa942831, *pte=00000000, *ppte=00000000
[   18.774749] Internal error: Oops: 17 [#1] SMP ARM
[   18.779663] Modules linked in:
[   18.782836] CPU: 0    Not tainted  (3.5.0-rc1-dirty #456)
[   18.788482] PC is at _od_resume_noirq+0x1c/0x78
[   18.793212] LR is at _od_resume_noirq+0x6c/0x78
[   18.797943] pc : [<c00307ec>]    lr : [<c003083c>]    psr: 20000113
[   18.797943] sp : ec3abe80  ip : ec3abdb8  fp : 00000006
[   18.809936] r10: ec1148b8  r9 : c08a48f0  r8 : c00307d0
[   18.815368] r7 : 00000000  r6 : 00000000  r5 : ec114800  r4 :
ec114808
[   18.822174] r3 : 00000000  r2 : 00000000  r1 : ec154fe8  r0 :
00000006
[   18.829010] Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM
Segment user
[   18.836456] Control: 10c5387d  Table: aa6b804a  DAC: 00000015
[   18.842437] Process sh (pid: 1139, stack limit = 0xec3aa2f0)
[   18.848358] Stack: (0xec3abe80 to 0xec3ac000)

DPI and SDI can be plain platform_devices. This patch changes the
registration from omap_device_register() to platform_device_add().

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: John Stultz <johnstul@us.ibm.com>
Tested-by: Jean Pihet <jean.pihet@newoldbits.com>