i2c: brcmstb: Allow enabling the driver on DSL SoCs

ARCH_BCM_63XX which is used by ARM-based DSL SoCs from Broadcom uses the
same controller, make it possible to select the STB driver and update
the Kconfig and help text a bit.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>