ARM: at91: remove useless Makefile.boot

Makefile.boot is not used anymore, remove it.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/mach-at91/Makefile.boot

-# Note: the following conditions must always be true:
-#   ZRELADDR == virt_to_phys(TEXTADDR)
-#   PARAMS_PHYS must be within 4MB of ZRELADDR
-#   INITRD_PHYS must be in RAM
-
-   zreladdr-y	+= 0x20008000
-params_phys-y	:= 0x20000100
-initrd_phys-y	:= 0x20410000