ARM: pxa: hx4700: Fix backlight PWM device number

Recent changes to PXA PWM support changed the PXA27X PWM device
numbering scheme.

The linux-3.5 PXA PWM driver followed the hardware numbering scheme for
the 4 PWMs, while the linux-3.6-rc1 PXA PWM driver has adopted a linear
numbering scheme:

Address		Hardware	3.5 pwm_id	3.6-rc1 pwm_id
0x40b00000	PWM0		0		0
0x40b00010	PWM2		2		1
0x40c00000	PWM1		1		2
0x40c00010	PWM3		3		3

The hx4700 backlight uses PWM1 at 0x40c00000. Consequently the pwm_id
must be changed from 1 to 2.

This patch fixes the backlight PWM device number and at the same time
moves from the legacy PWM API (pwm_id) to the new PWM API (pwm_lookup).

Signed-off-by: Paul Parsons <lost.distance@yahoo.com>
Cc: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>