Add support for the %pV format-specifier which allows printing a
struct va_format. This is used by the Linux kernel for recursive
printf() formatting and is needed by the ext4l filesystem driver.
Add the struct to include/linux/printk.h to match the kernel location.
Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>