raspberrypi4.rst: add supported RPi4 board

Added new board support (RaspberryPi4) for OpenHarmony.
fixes #37

Signed-off-by: Sudhesh Kumar <sudhesh.kumar.kuppusamy@huawei.com>
2 jobs for !106 with raspberrypi-docs in 23 seconds (queued for 2 seconds)
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Status Job ID Name Coverage
  Compliance
passed #14849
dco

00:00:11

 
  Build
passed #14850
build-docs

00:00:12