driver: mpam: deal with unused variable prot

In commit de8f520a

 (driver:mpam: Fix the compilations issues when
ACPI is not selected) introduced compiler warning below, add #ifdef
statement around variable 'prot' and replace ACPI with CONFIG_ACPI to
solve this problem.

drivers/platform/mpam/mpam_devices.c: In function 'mpam_msc_drv_probe':
drivers/platform/mpam/mpam_devices.c:1598:18: warning: unused variable 'prot' [-Wunused-variable]
 1598 |         pgprot_t prot;
      |                  ^~~~

Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>