docs(msm8916): document new platforms

Document the new platform build options for the MSM8916 port which now
supports multiple similar Qualcomm SoCs:

  - Snapdragon 410 (PLAT=msm8916) as before
  - Snapdragon 615 (PLAT=msm8939)
  - Snapdragon 210 (PLAT=msm8909)
  - Snapdragon X5 Modem (PLAT=mdm9607)

The latter two use AArch32-only ARM Cortex-A7 cores that only support
using BL32/SP_MIN and not BL31 on AArch64.

Change-Id: I9fffe60dd0ad2acc18f006f11e91854b9e8dcb8f
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
2 files changed