riscv/efi_stub: Add 64bit boot-hartid support on RV64

The boot-hartid can be a 64bit value on RV64 platforms but
the "boot-hartid" in DT is assumed to be 32bit only.

Detect the size of the "boot-hartid" in DT and use 32bit or 64bit
read appropriately.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Link: https://lore.kernel.org/r/20220527051743.2829940-6-sunilvl@ventanamicro.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>