efi_loader: disable GRUB_ARM32_WORKAROUND on ARCH_SUNXI

GRUB_ARM32_WORKAROUND can be disabled on ARCH_SUNXI as the Allwinner SoCs
only have a level 2 cache controlled via CP15 and not an architecturally
defined cache. Having the cache available speeds up booting Linux.

On ARCH_BCM283X it is already disabled via rpi_2_defconfig. But let's move
this setting to Kconfig.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>