efi/cper: Fix cper_arm_ctx_info alignment

According to the UEFI Common Platform Error Record appendix, the
processor context information structure is a variable length structure,
but "is padded with zeros if the size is not a multiple of 16 bytes".

Currently this isn't honoured, causing all but the first structure to
be garbage when printed. Thus align the size to be a multiple of 16.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>