imx: imx_snvs: Define a SNVS header and memory map

imx: imx_snvs: Define a SNVS header and memory map

This commit defines two things.

- The basic SNVS memory map. At the moment that is total overkill for the
  permission bits we need to set inside the SNVS but, for the sake of
  completeness define the whole SNVS area as a struct.

- The bits of the HPCOMR register

  A permission fix will need to be applied to the SNVS block prior to
  switching on TrustZone. All we need to do is waggle a bit in the HPCOMR
  register. To do that waggle we first need to define the bits of the
  HPCOMR register.

- A imx_snvs_init() function definition

  Declare the snvs_init() function so that it can be called from our
  platform setup code.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>