ARM: P2V: separate PHYS_OFFSET from platform definitions

This uncouple PHYS_OFFSET from the platform definitions, thereby
facilitating run-time computation of the physical memory offset.

Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Magnus Damm <damm@opensource.se>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Wan ZongShun <mcuos.com@gmail.com>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Acked-by: Jiandong Zheng <jdzheng@broadcom.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>