power: reset: Add MFD_SYSCON depends for brcmstb

The Broadcom STB reboot driver depends on MFD_SYSCON, it uses
syscon_regmap_lookup_by_phandle() which will not lookup syscon phandles
if MFD_SYSCON is disabled, and instead will return -ENOSYS since it is
turned into an inline stub.

Fixes: 030494e7 ("power: reset: Add reboot driver for brcmstb")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>