rcar_gen3: plat: Fix BL2 size check

Rename BL2_LIMIT to BL2_IMAGE_LIMIT and BL2_SYSRAM_LIMIT to BL2_LIMIT to
correctly set BL2_LIMIT value. Set correct DEVICE_SRAM_BASE to match the
hardware. Use BL2_END in rcar_configure_mmu_el3() to mark the cacheable
BL2 area.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>