irqchip/irq-brcmstb-l2: Avoid saving mask on shutdown

The interrupt controller shutdown path does not need to save the mask of
enabled interrupts because the next state the system is going to be in is
akin to a cold boot, or a kexec'd kernel. Saving the mask only makes sense
if the software state needs to preserve the hardware state across a system
suspend/resume cycle.

As an optimization, and given that there are systems with dozens of such
interrupt controller, save a "slow" memory mapped I/O read in the shutdown
path where no saving/restoring is required.

Reported-by: Tim Ross <tim.ross@broadcom.com>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240424175732.1526531-1-florian.fainelli@broadcom.com