arch:exynos: boot mode: add get_boot_mode(), code cleanup

arch:exynos: boot mode: add get_boot_mode(), code cleanup

This patch introduces code clean-up for exynos boot mode check.
It includes:
- removal of typedef: boot_mode
- move the boot mode enum to arch-exynos/power.h
- add bootmode for sequence: eMMC 4.4 ch4 / SD ch2
- add new function: get_boot_mode() for OM[5:1] pin check
- update spl boot code

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Changes v5:
- exynos: boot mode: add missing bootmode (1st:EMMC 4.4 / 2nd:SD ch2)

Changes v6:
- none

changes v7:
- change boot mode name: BOOT_MODE_MMC to BOOT_MODE_SD
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>