arm64: efi: add missing frame pointer assignment

The prologue of the EFI entry point pushes x29 and x30 onto the stack but
fails to create the stack frame correctly by omitting the assignment of x29
to the new value of the stack pointer. So fix that.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>