power: reset: at91-reset: Optimize at91_reset()

power: reset: at91-reset: Optimize at91_reset()

This patch adds a small optimization to the low-level at91_reset()
function, which includes:
- Removes the extra branch, since the following store operations
  already have proper condition checks.
- Removes the definition of the clobber register r4, since it is
  no longer used in the code.

Fixes: fcd0532fac2a ("power: reset: at91-reset: make at91sam9g45_restart() generic")
Signed-off-by: Alexander Shiyan <eagle.alexander923@gmail.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20250307053809.20245-1-eagle.alexander923@gmail.com
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>