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Current code assumes `SCP_COM_SHARED_MEM_BASE` as the base address
for BOM/SCPI protocol between AP<->SCP on all CSS platforms. To
cater for future ARM platforms this is made platform specific.
Similarly, the bit shifts of `SCP_BOOT_CONFIG_ADDR` are also made
platform specific.

Change-Id: Ie8866c167abf0229a37b3c72576917f085c142e8