arm64: defconfig: enable ARCH_BCM

Commit 96796c91 ("arm64: Kconfig.platforms: Re-organized Broadcom
menu") introduced a new symbol ARCH_BCM and made all of the Broadcom
SoCs Kconfig depend on it, but did not enable it in the defconfig.
Thus, now the defconfig doesn't include support for any Broadcom
architectures anymore. Fix it.

Fixes: 96796c91 ("arm64: Kconfig.platforms: Re-organized Broadcom menu")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220906033957.4377-1-f.fainelli@gmail.com

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