docs: platform/generic: Add details about IPI and timer expectations

The generic platform provides IPI and timer functionality based on
DT node provided in the FDT passed by previous booting stage. This
patch updates generic platform documentation about IPI and timer
expectations.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>