soc: brcmstb: Correct CPU_CREDIT_REG offset for Brahma-B53 CPUs

On Broadcom Brahma-B53 CPUs, the CPU_CREDIT_REG offset got moved to
0x0b0 instead of 0x184, correct this such that we correcty
enable/disable write-pairing for these chips.

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