platform: Allow platforms to specify heap size

We extend struct sbi_platform and struct sbi_scratch to allow platforms
specify the heap size to the OpenSBI firmwares. The OpenSBI firmwares
will use this information to determine the location of heap and provide
heap base address in per-HART scratch space.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>