powerpc: Make CONFIG_RTAS_PROC depend on CONFIG_PROC_FS

We are getting build errors with CONFIG_PROC_FS=n:

arch/powerpc/kernel/rtas_flash.c
   In function 'rtas_flash_init':
  745:33: error: unused variable 'f' [-Werror=unused-variable]

But rtas_flash.c should not be built when CONFIG_PROC_FS=n, beacause all
it does is provide a /proc interface to the RTAS flash routines.

CONFIG_RTAS_FLASH already depends on CONFIG_RTAS_PROC, to indicate that
it depends on the RTAS proc support, but CONFIG_RTAS_PROC does not
depend on CONFIG_PROC_FS. So fix that.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>