arm: spear: Use PUSH/POP mnemonics when relevant

arm: spear: Use PUSH/POP mnemonics when relevant

Quoting ARM "RealView Compilation Tools Assembler Guide v4.0":

        PUSH and POP are synonyms for STMDB and LDM (or LDMIA), with
        the base register sp (r13), and the adjusted address written
        back to the base register.
	PUSH and POP are the preferred mnemonic in these cases.

Let's follow this recommandation to ease the reading and substitute
LDMIA/STMDB operations with PUSH/POP mnemonics.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Stefan Roese <sr@denx.de>