spl: Use standard FIT entries

spl: Use standard FIT entries

SPL is creating fit-images DT node when loadables are recorded in selected
configuration. Entries which are created are using entry-point and
load-addr property names. But there shouldn't be a need to use non standard
properties because entry/load are standard FIT properties. But using
standard FIT properties enables option to use generic FIT functions to
descrease SPL size. Here is result for ZynqMP virt configuration:
xilinx_zynqmp_virt: spl/u-boot-spl:all -82 spl/u-boot-spl:rodata -22 spl/u-boot-spl:text -60

The patch causes change in run time fit image record.
Before:
fit-images {
        uboot {
                os = "u-boot";
                type = "firmware";
                size = <0xfd520>;
                entry-point = <0x8000000>;
                load-addr = <0x8000000>;
        };
};

After:
fit-images {
        uboot {
                os = "u-boot";
                type = "firmware";
                size = <0xfd520>;
                entry = <0x8000000>;
                load = <0x8000000>;
        };
};

Replacing calling fdt_getprop_u32() by fit_image_get_entry/load() also
enables support for reading entry/load properties recorded in 64bit format.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>