[ARM] Clean up KERNEL_RAM_ADDR

Clean up the KERNEL_RAM_ADDR stuff in arch/arm/kernel/head.S to
make it clearer what's referring to what.  In doing so, remove
the usage of __virt_to_phys(), which is not guaranteed to be
something that the assembler can parse.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>