PXA: fix MDREFR[APD] bit setting

pxa_mem_setup macro use r6 to store CONFIG_SYS_MDREFR_VAL during memory
initialization. This reg is modified during execution of pxa_wait_ticks.
Later we use r6 to setup MDREFR[APD] bit. As result MDREFR[APD] is always
zero.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@gmail.com>