efi_loader: Change efi_dp_from_mem() to use size

efi_loader: Change efi_dp_from_mem() to use size

All call sites are using size rather than end addresses,
so instead - as previously done - calculating an end address
everywhere, just modify the function to use size and internally
calculate the end address

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc: Patrick Wildt <pwildt@google.com>
Signed-off-by: Moritz Fischer <moritzf@google.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>