bus: ti-sysc: Fix using configured sysc mask value

[ Upstream commit e212abd452a4af3174fcd469d46656f83e135a19 ]

We have cases where there are no softreset bits like with am335x lcdc.
In that case ti,sysc-mask = <0> needs to be handled properly.

Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>