firmware: exynos-acpm: Correct kerneldoc and use typical np argument name

firmware: exynos-acpm: Correct kerneldoc and use typical np argument name

Correct kerneldoc warnings after commit a8dc26a0ec43 ("firmware:
exynos-acpm: introduce devm_acpm_get_by_node()") changed the function
prototype:

  exynos-acpm.c:672: warning: Function parameter or struct member 'acpm_np' not described in 'acpm_get_by_node'
  exynos-acpm.c:672: warning: expecting prototype for acpm_get_by_phandle(). Prototype was for acpm_get_by_node() instead

While touching the lines, change the name of device_node pointer to
'np' to match convention.

Fixes: a8dc26a0ec43 ("firmware: exynos-acpm: introduce devm_acpm_get_by_node()")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202504222051.7TqaSQ48-lkp@intel.com/
Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://lore.kernel.org/r/20250424203308.402168-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>