mpc83xx: retain POR values of non-configured ACR, SPCR, SCCR, and LCRR bitfields

some LCRR bits are not documented throughout the 83xx family RMs.
New board porters copying similar board configurations might omit
setting e.g., DBYP since it was not documented in their SoC's RM.

Prevent them bricking their board by retaining power on reset values
in bit fields that the board porter doesn't explicitly configure
via CONFIG_SYS_<registername>_<bitfield> assignments in the board
config file.

also move LCRR assignment to cpu_init_r[am] to help ensure no
transactions are being executed via the local bus while CLKDIV is being
modified.

also start to use i/o accessors.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>