[ARM] pxa: introduce pxa{25x,27x,300,320,930}.h for board usage

Considering the header mess ATM, it is not always possible to include
the correct header files within board code. Let's keep this simple:

  <mach/pxa25x.h>  - for pxa25x based platforms
  <mach/pxa27x.h>  - for pxa27x based platforms
  <mach/pxa300.h>  - for pxa300 based platforms
  <mach/pxa320.h>  - for pxa320 based platforms
  <mach/pxa930.h>  - for pxa930 based platforms

NOTE:

1. one header one board file, they are not compatible (i.e. they have
   conflicting definitions which won't compile if included together).

2. Unless strictly necessary, the following header files are considered
   to be SoC files use _only_, and is not recommended to be included in
   board code:

    <mach/hardware.h>
    <mach/pxa-regs.h>
    <mach/pxa2xx-regs.h>
    <mach/pxa3xx-regs.h>
    <mach/mfp.h>
    <mach/mfp-pxa2xx.h>
    <mach/mfp-pxa25x.h>
    <mach/mfp-pxa27x.h>
    <mach/mfp-pxa3xx.h>
    <mach/mfp-pxa300.h>
    <mach/mfp-pxa320.h>
    <mach/mfp-pxa930.h>

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