efi_loader: use correct type for AllocatePages, AllocatePool

Use enum efi_memory_type and enum_allocate_type in the definitions of the
efi_allocate_pages(), efi_allocate_pool().

In the external UEFI API leave the type as int as the UEFI specification
explicitely requires that enums use a 32bit type.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>