ARM: EXYNOS: Do not calculate boot address twice

Commit b3205dea8fbf ("ARM: EXYNOS: Map SYSRAM through generic DT
bindings") introduced local variable boot_reg where boot address from
cpu_boot_reg() call is stored. Re-use it instead calling cpu_boot_reg()
again.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Sachin Kamat <sachin.kamat@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>