powerpc: Fix typo in uImage target

Commit e32e78c5


(powerpc: fix build with make 3.82) introduced a
typo in uImage target and broke building uImage:

make: *** No rule to make target `uImage'.  Stop.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: stable <stable@kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>