power: reset: Allow selecting POWER_RESET_BRCMSTB on ARM64

Since commit 37eb56dc ("arm64: Add Broadcom Set Top Box Kconfig
entry point") we have ARCH_BRCMSTB also visible on ARM64 platform, yet
this reboot driver was not selectable, so fix that.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>