ramips: fix partition layout of hiwifi hc5x61

Changes:
 * Increase "oem" partition size from 0x10000 to 0x20000
 * Correct partition lables, synchronize with official firmware

Evidence:
It should be the same as hiwifi hc5x61a and the fact indeed the
case. Here is part of dmesg boot log read from official firmware:
[    1.470000] Creating 7 MTD partitions on "raspi":
[    1.470000] 0x000000000000-0x000000030000 : "u-boot"
[    1.480000] 0x000000030000-0x000000040000 : "hw_panic"
[    1.490000] 0x000000040000-0x000000050000 : "Factory"
[    1.490000] 0x000000fc0000-0x000000fe0000 : "oem"
[    1.500000] 0x000000fe0000-0x000000ff0000 : "bdinfo"
[    1.510000] 0x000000ff0000-0x000001000000 : "backup"
[    1.510000] 0x000000050000-0x000000fc0000 : "firmware"

Signed-off-by: Shiji Yang <yangshiji66@qq.com>