Documentation: EXYNOS: Update bootloader interface on exynos542x

Update the documentation about:
1. Usage of PMU_SPARE2 register.
   Bootloaders on Exynos542x-based boards often use the register
   PMU_SPARE2 (0x908) in the same way as on Exynos3250: as a indicator
   the secondary CPU was booted on. The bootloader will set this value
   to non-zero, after sucessfull power up of secondary CPU. In the same
   time this booted CPU will stuck (spin) waiting for software reset.
2. Exynos542x entry address for secondary CPU boot up after system
   suspend (with MCPM enabled and in non-secure mode).

See arch/arm/mach-exynos/mcpm-exynos.c for source code.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>