lib: Simplify sbi_platform timer_init() hooks

Instead of having separate timer_init() hooks for cold and
warm boot, this patch updates struct sbi_platform to have just
one timer_init() hook. The type of boot (cold or warm) is now
a boolean flag parameter for the updated timer_init() hook.

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