ARM: 8531/1: turn init_new_context into an inline function

Almost all architectures define init_new_context() as a function,
but on ARM, it's a macro and that causes a compiler warning when
its return code is not used:

drivers/firmware/efi/arm-runtime.c: In function 'efi_virtmap_init':
arch/arm/include/asm/mmu_context.h:88:34: warning: statement with no effect [-Wunused-value]
 #define init_new_context(tsk,mm) 0
drivers/firmware/efi/arm-runtime.c:47:2: note: in expansion of macro 'init_new_context'
  init_new_context(NULL, &efi_mm);

This changes the definition into an inline function, which gcc does
not warn about.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>