platform: generic: Add support for specify coldboot harts in DT

Added support for the generic platform to specify the set of coldboot
hart in DT. If not specified in DT, all harts are allowed to coldboot
as before.

The functions related to sbi_hartmask are not available before coldboot,
so I used bitmap, and added a new bitmap_test() function to test whether
a certain bit of the bitmap is set.

Signed-off-by: Cheng Yang <yangcheng.work@foxmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>