boards: mpfs_icicle: Set CONFIG_MP_MAX_NUM_CPUS=1 for non-SMP build variant

Besides E51 and U54 SMP variants, one can target
a single core of U54 for testing purposes.

There is already CONFIG_RV_BOOT_HART=1 that specifies
the starting HART ID for the SMP core.
Set CONFIG_MP_MAX_NUM_CPUS=1 as well to limit
the number of physical CPUs visible at build time
so that Zephyr code tries running only on the selected one.

Signed-off-by: Marek Slowinski <mslowinski@antmicro.com>