xen/arm: introduce new helper parse_static_mem_prop and acquire_static_memory_bank

xen/arm: introduce new helper parse_static_mem_prop and acquire_static_memory_bank

Later, we will introduce assign_static_memory_11 for allocating static
memory for direct-map domains, and it will share a lot common codes with
the existing allocate_static_memory.

In order not to bring a lot of duplicate codes, and also to make the whole
code more readable, this commit extracts common codes into two new helpers
parse_static_mem_prop and acquire_static_memory_bank.

Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>