part: efi: Fix offset

Both the config option and the DT options specify the offset to set the GPT
at in bytes, yet the code treats those values as block numbers.

Fix that.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>