mpc85xx: change CPU_TYPE from 8540 to 8548

mpc85xx: change CPU_TYPE from 8540 to 8548

All subtargets as p1010, p1020, p2020 uses e500v2 cores [1] [2] [3] [4],
 so let's force cpu type to 8545 for all mpc85xx target even though
there might exist device with e500v1, but there is no such router supported in OpenWrt
and because of that it does not make sense to to set CPU_TYPE for each subtarget.

All MPC85xx SoC can be found here:
https://www.nxp.com/products/processors-and-microcontrollers/legacy-mpu-mcus/powerquicc-processors:POWERQUICC_HOME#powerquicc-iii-mpc85xx

[1] https://www.nxp.com/docs/en/application-note/AN2807.pdf
[2] https://www.nxp.com/docs/en/fact-sheet/QP1010FS.pdf
[3] https://www.nxp.com/docs/en/product-brief/P1020PB.pdf
[4] https://www.nxp.com/docs/en/fact-sheet/QP20XXFS.pdf

Co-authored-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Šimon Bořek <simon.borek@nic.cz>