uboot-imx

omap: Remove hardcoding of mmu section shift to 20

As of now the mmu section shift is hardcoded to 20 but with LPAE
coming into picture this can be different. Hence replacing 20 with
MMU_SECTION_SHIFT macro.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>