ARM: B15: Update to support Brahma-B53

The B53 CPU design supports up to 8 processors, which moved the RAC_FLUSH_REG
offset 0x4 bytes below to make room for a RAC_CONFIG2_REG to control RAC
settings for CPU4-7.

Lookup the processor type (B15 or B53) and adjust the RAC_FLUSH_REG offset
accordingly, if we do not know the processor, bail out.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>