lib: Simplify sbi_platform ipi_init() hooks

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

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