[ARM] pxa: separate definitions from pxa-regs.h and remove it finally

The remaining registers are separated into:

   - <mach/regs-ost.h>
   - <mach/regs-rtc.h>
   - <mach/regs-intc.h>

and then we can remove pxa-regs.h completely. Instead of #include this
file, let's:

1. include the specific <mach/regs-*.h> with care (if that's absolutely
   necessary)

2. define the registers in the driver, make cleanly defined API to expose
   the register access to external with sufficient reason

Signed-off-by: Eric Miao <eric.miao@marvell.com>