arm64: Kconfig.platforms: Re-organized Broadcom menu

There are now multiple Broadcom SoCs supported so group them under their
own menu such that the selection is visually more appealing and we can
easily add new platforms there in the future. This allows us to move
ARCH_BRCMSTB back to its siblings.

No functional changes introduced.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220829173830.3567047-2-f.fainelli@gmail.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>