[ARM] pxa: move common GPIO handling code into plat-pxa

1. add common GPIO handling code into [arch/arm/plat-pxa]

2. common code in <mach/gpio.h> moved into <plat/gpio.h>, new processors
   should implement its own <mach/gpio.h>, provide the following required
   definitions and '#include <plat/gpio.h>' in the end:

   - GPIO_REGS_VIRT for mapped virtual address of the GPIO registers'
     physical I/O memory

   - macros of GPLR(), GPSR(), GPDR() for constant optimization for
     functions gpio_{set,get}_value() (so that bit-bang code can still
     have tolerable performance)

   - NR_BUILTIN_GPIO for the number of onchip GPIO

   - definitions of __gpio_is_inverted() and __gpio_is_occupied(), they
     can be either macros or inlined functions

Signed-off-by: Eric Miao <eric.miao@marvell.com>
diff --git a/arch/arm/mach-pxa/Makefile b/arch/arm/mach-pxa/Makefile
index 70b4657..c80e1ba 100644
--- a/arch/arm/mach-pxa/Makefile
+++ b/arch/arm/mach-pxa/Makefile
@@ -4,7 +4,7 @@
 
 # Common support (must be linked before board specific support)
 obj-y				+= clock.o devices.o generic.o irq.o \
-				   time.o gpio.o reset.o
+				   time.o reset.o
 obj-$(CONFIG_PM)		+= pm.o sleep.o standby.o
 
 ifeq ($(CONFIG_CPU_FREQ),y)