board_f.c: Ensure 16 alignment of start_addr_sp and reserved memory

Add a function reserve_stack_aligned() to reserved memory with 16 bits
alignment after the stack pointer (gd->start_addr_sp) and use this new
function in board_f.c to reserve all the memory area (malloc, board, gd,
fdt, bootstage, stacks).

This 16 byte alignment is needed for cast on struct pointer
for the reserved memory, for example:
+ x86_64 ABI: https://reviews.llvm.org/D30049

: 16 bytes
+ ARMv8 Instruction Set Overview: quad word, 16 bytes

An other alignment value could be needed for other architecture.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Stephen Warren <swarren@nvidia.com>