CONFIG_MACH_SPEAR300 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

SPEAr300 Machine support with Device Tree

modulename: spear300.ko

configname: CONFIG_MACH_SPEAR300

Linux Kernel Configuration
└─>SPEAr3xx Implementations
└─>SPEAr300 Machine support with Device Tree
In linux kernel since version 2.6.35 (release Date: 2010-08-01)  
Supports ST SPEAr300 machine configured via the device-tree

source code:
arch/arm/mach-spear3xx/spear300.c

depends
CONFIG_ARCH_SPEAR3XX |
| CONFIG_PLAT_SPEAR

is selected by
CONFIG_PINCTRL_SPEAR300