platform: Remove PLAT_HART_COUNT and PLAT_HART_STACK_SIZE

The PLAT_HART_COUNT and PLAT_HART_STACK_SIZE are only used
by platform code so no need of exposing these to everyone
by adding it to platform-cppflags-y.

Signed-off-by: Anup Patel <anup.patel@wdc.com>