Currently the SCU registers are mapped in SoC-specific code, using
different methods, all involving the static mapping set up from
machine_desc.map_io():
  - On emev2, a static (non-identity) mapping is used, with ioremap().
    As the static mapping uses the MT_DEVICE type, ioremap() reuses it,
    and the returned virtual address is suitable for passing to
    shmobile_smp_hook(),
  - On sh73a0 and r8a7779, a static identity mapping is used, with the
    legacy IOMEM() macro.
    As the static mapping uses the MT_DEVICE_NONSHARED type, replacing
    IOMEM() by ioremap() would create a new mapping, whose virtual
    address cannot be passed to shmobile_smp_hook().

Move the mapping of the SCU registers from SoC-specific code to common
code, always using ioremap(). To work in the absence of a static
mapping, this requires passing the physical SCU base address to
shmobile_smp_hook().
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>