efi: Fix compiler warnings (unused, const, type)

This patch fixes a few compiler warning in the efi code for unused
variable, discarding const qualifier and wrong pointer type:

drivers/firmware/efi/fdt.c|66 col 22| warning: unused variable ‘name’ [-Wunused-variable]
drivers/firmware/efi/efi.c|368 col 3| warning: passing argument 3 of ‘of_get_flat_dt_prop’ from incompatible pointer type [enabled by default]
drivers/firmware/efi/efi.c|368 col 8| warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>